Multiple Location VOIP phone system

First of all, I dont know much about VOIP. So my question is really can and how can this be done in a general sense.

I want to setup a voip phone system that will operate from a central office. There will also be a large number of remote phones that will have their own phone numbers. These remote phones will be located in remote households or offices. If someone tries to ring one of the remote phones and no one is there to answer the call it should be redirected to the central office with the relevant information displayed so that the reception can take the call and know who it was originally intended for.

Hopefully that makes sense.
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grbladesConnect With a Mentor Commented:
I use Asterisk ( and that would be a fairly easy thing to do.
In the dialplan you just call the extension of the phone and if it times out or the phone is unavailable then the dialplan jumps to step 2.
IP phones display the callerID name and number. Therefore for step 2 you can leave the callerid number alone but set the name. For example if the call was for 'john smith' then you could set the callerid name to 'DIVERT John Smith'.
Step 3 you call the office phones. The phone will show the callerid name and number. For the callerid name it will then show 'DIVERT John Smith' so the person knows the call was diverted from that persons number.
feptiasConnect With a Mentor Commented:
Would your voip phone system be replacing an existing PBX or integrating with it?
When someone rings the remote phones, might they be calling from outside your organisation or only from extensions on your voip phone system?

Potentially, this could be quite a complex system especially if it has to integrate seamlessly with an existing PBX and be able to route calls from the PSTN to specific remote user phones based on the dialled number, handle outbound calls from the remote users etc.

In a general sense, what you require is a SIP Registrar and Proxy server that would be located at the central office - it must be connected to the Internet on a static IP address. The remote phones would then register with that server so their location would be known. Calls for the remote phones would initially have to be routed to the Proxy server which would forward the call to the remote phone and could monitor if the call is answered, busy etc. The proxy server could then take appropriate action for unanswered calls - e.g. divert the call elsewhere. Asterisk might be suitable for this role, but in my opinion it would only be "fairly easy" once you had mastered Asterisk.
SM17CHAuthor Commented:
-Yes, people would be calling the remote phones from outside the organisation.
-There is no PBX installed at the moment to worry about.

So basically it works like this:
-Install a SIP Registrar/ Proxy server like asterix
-Install Soft Phones at all the various locations and register them with the server
-Setup phone locations, ID's on server
-Setup call redirection on server

Can the server just be a box with the appropriate software installed that is connected to the organisations network (therefore the internet)?

How do you get the physical phone numbers for each phone in the first place? Do you rent them from a voip company? How does that work?

Thanks again,\
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>So basically it works like this:
Yes that is basically correct. Asterisk isnt a proxy server but if you set it up correctly you wont need to use a proxy anyway.

Yes the the server is just a normal PC. Asterisk can run on Windows but the best way is to run it as designed on a Linux system. If you are not familar with Linux then you can use Trixbox ( which is a bootable CD which installs Linux, Asterisk and lots of web tools so you can configure it using a web browser.

To get telephone numbers you can either buy telephony cards for the server and connect it to your analogue/PRI lines or you can rent numbers from voip providers on the Internet. I recently helped a customer configure their server and they were using Voxbone ( to get local numbers in about 40 countries. It worked very well.
In asterisk all you need to do is configure the provider listing their username/password and when a call is made to one of your numbers they send the call over VoIP to your box and provide information on which number is dialed and the callerid information.
With VoIP you need to be carefull over the quality of the internet connection and the bandwidth used. You can configure QOS (Quality Of Service) on the network but this can only go so far if the internet connection is also used for web browsing. Normally I recommend that people get a 2nd internet connection just for VoIP as it is really the only way to guarantee the voip traffic does not get disturbed.

Then you need to consider bandwidth. You need to work out how many calls will be going over the internet connection. A call coming in and then going back out to someones home phone will count as two calls so this uses up bandwidth quickly. For the highest quality each call consumes 80kbps of bandwidth so if you have 5 active calls all between the voip provider and people outside the office that is 800kbps of bandwidth. There are ways to reduce this by using a feature called trunking between the office and the voip provider and also using higher compression on the voice.
Which you go for will depend on the number of users and how expensive your internet connection is.
SM17CHAuthor Commented:
Can you please elaborate on this?

"Yes that is basically correct. Asterisk isnt a proxy server but if you set it up correctly you wont need to use a proxy anyway."

What are the benifits of a proxy server?
What do you mean by set it up properly?

Thanks to everyones help so far. I really appreciate it.
The most popular protocol used by VoIP is SIP. SIP is a signalling protocol used to setting up the voice call. It uses another protocol called RTP for transmitting the actual voice. The problem is that SIP can setup a call and say something like "I am waiting for your voice packets on udp port 10000". Thats fine within a local network but if you are talking to another machine over the internet it cannot send anything to because it is a private IP address so you end up getting one way audio.
A proxy server allows a client to register and it detects these sorts of problems and tries to work around them.
Asterisk however can be configured with the IP range of your internal network and the IP address of your internet connection. So it works out what it is talking to and always uses the correct IP address in the SIP messages. There is then no need to use a proxy server.
Asterisk can also use its own protocol called IAX which is much simpler and does not have all these inherent problems that SIP does. Its ideal for people traveling as it works in places like hotels where often SIP does not.
By set up properly I mean the above mentioned IP addresses are correctly configured and the port range used by RTP is forwarded on any firewall you are using.
I only just have seen the latest comments added today. SIP and Asterisk are so flexible, it means there is more than one approach that you could use for this project. What is best probably depends on how your voip system is going to be linked to the PSTN, what telephony facilities you require in the central office that you mentioned, the balance between number of inbound calls and number of outbound calls, the balance between "internal" calls (remote workers would count as internal extensions in your case) and calls to external PSTN numbers and International numbers. If you plan to put most of your telephony traffic on VoIP over the Internet to a voip provider then you must be quite sure the reliability and voice quality for those calls is up to the standard you require.

Personally, I would never recommend that a business has its entire telephony system connected to the outside world using only VoIP. I would always recommend keeping some kind of conventional line connecting your office to the PSTN, at least as a backup for emergencies. (Do you agree grblades?).

Once you have made the decision to keep a conventional line (perhaps even a PRI line capable of carrying a good number of simultaneous calls), then you have got the option to provide your own gateway between the PSTN and the internal VoIP phone network. Asterisk is very good in this role. You could then make it so all inbound calls to the company's published numbers arrive on the PRI line rather than using your VoIP service provider to handle inbound calls - the cost of the PRI line will be higher, but you will be assured of better reliability and voice quality for people calling your company. You can still use the VoIP service provider to route outbound calls more cheaply, especially to International numbers. Again, Asterisk is great in this role and can act as a "least cost router" choosing automatically to use the VoIP circuit for outbound calls and with failover to the PRI circuit should the VoIP connection fail (or maybe use prefix 9 for a VoIP outbound route and prefix 8 for a PRI outbound route so giving the caller the abilty to choose).

If the Asterisk server is acting as the registrar for all those remote phones then it could easily route inbound calls to the appropriate remote phone, either using an automated attendant (easy to implement within Asterisk) or using a unique DDI number for each remote user. Internal calls from the office to the remote users would be equally easily handled. Remote users could call each other, call phones in the office or make calls to the PSTN via the office Asterisk server provided you set things up with a sensible numbering plan. All internal calls, even to/from remote users, would be free in this situation.

That is one solution. There are other ways you could do it. The exact meaning of the terminology ("Proxy server", "Registrar server", "B2BUA" and all that) is not too important - it is more important that you decide if you want a "home grown" solution like Asterisk or go for a manufactured iPBX from someone like Cisco, Avaya, Mitel, Nortel, 3-Com etc. The former will require internal support and the latter you will need to pay for a maintenance contract. Also decide how much you trust VoIP over the Internet to handle your company's telephony traffic - you are trading cost against reliability/quality, but this partly depends on the status of your Internet connections which you may regard as very reliable and having plenty of guaranteed bandwidth.

Enough of my rant. Hope this helps.
Yes I do normally recommend that you have conventional lines aswell. Mostly its a case of reliability. Phone services tend to be extremely reliable and internet connections far less so. So if you put all the calls over the internet they you are going to end up with a less reliable system typically.
This may be fine for a lot of people. VoIP providers may have their own routing options so if asterisk does not acknowledge the call due to connectivity issues a recorded message could be played back informing the use of an alternative number such as a mobile phone.

Personally in the UK the only way to get a reliable connection is to get a leased line or some frame-relay type of internet connection. Thats expensive so we just kept our PRI/E1 telephone circuit and pay about 360 GBP per month for it which is cheaper than an high quality internet connection. We have a dedicated 512kbps SDSL internet connection just for voip but that is for salemen to be able to make call from hotels etc... while traveling.

Thanks. Your comments reminded me to mention about faxes and modems. Due to the compression that is used over VoIP lines they are incredibly poor at transferring any sort of modem calls. You would be lucky to get a 9600 modem connection over a voip line. For faxing there is a protocol called T.38 where the fax is decoded before it is sent over voip and then re-encoded afterwards so you can fax over voip but it requires additional configuration.
Therefore you will most certenly want a few analogue lines for any faxes or modems.
kode99Connect With a Mentor Commented:
It is probably worth considering or at least comparing a hosted or virtual setup.  The kind of operation talked about is really a fairly typical voip PBX application so as mentioned, there are a number of hardware choices that will work fine.  

I myself, like grblades,  also favor a in-house approach with lots of options like Asterisk offers.  At the same time though I do think it is important to be aware of the cost and pros/cons of setting up this service through a provider.  This approach can get a system setup with minimal technical know how and reduces you to just having a connection with sufficient bandwidth for any given location where service needs to be.  This does offer some advantages,  for example if you main office internet did fail,  all your remote sites would be unaffected.

Here's an example of a hosted provider geared for business,

Overall probably some useful information though.  The cost comparisons are geared with comparing there service to something like a Cicso PBX installation,  which is going to be higher than an Asterisk system,  even a turnkey Asterisk setup.

How things would work out does somewhat depend on the scale of operation we are talking about and possibly specific features needed.  Also things like expected growth and the level of in-house technical expertise required to maintain operations.

SM17CHAuthor Commented:
Thanks everybody, appreciate all your input. It has given me alot to think about :)
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