VoIP - Trixbox / Asterix - PSTN and ISDN system proposal. Feasible?

I was hoping to get some feedback on a Voip system based on trixbox. This is the scenario:

**Internal Offices**
-Main office has 4 phones and 2 or 3 PSTN lines.
-Local offices (within 50m of main building) will each have their own phone number which will be allocated to them by the main office. Roughly 30 different numbers so ISDN is the best way to go here.
-PSTN lines and ISDN lines are connected to a Trixbox server.
-Softphones registered with the sever.
-Incoming calls automatically route to the correct phone, if it is busy or unanswered it will route to the main office reception and display caller info.
-Outgoing calls will route over PSTN/ISDN lines depending on which handset is used.

**External Offices**
-There are a number of external offices that currently have regular landlines and handsets.
-We want to install an ATA box at each office and route calls back to the trixbox, and then to main office when a call is unanswered or phone is busy. Call info will display on main office handset.
-Preferably office would use regular handset
-Outgoing calls route over PSTN network
-Calls being transferred will route over the internet to the trixbox.

Main office will have a 512/512 ADSL connection
External Offices will have at least 512/256 adsl connection

-What issues do you see arising with this system?
-Is it a big deal to setup PSTN and ISDN lines on trixbox? What is invloved in getting them to route to the correct phone?
-How do we connect fax lines?

I think that will do for now

Thanks for your time, I really appreciate it
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SM17CHAuthor Commented:
when i say softphones I mean IP Handsets.
So the main office will have an ISDN line and 3 PSTN lines. How will the calls go to the other internal offices?
Since they are 50m away I assume you are just going to send them over a normal cat5 network connection as it supports connections up to 100m between switches.

512/256 ADSL is limited by the 256k speed. For a high quality voip connection you will be able to support 3 simultaneous calls only.

Having a device at the remote office which will answer the line and pass it to the head office is not entirely straight forward. You could have the box immediatly call the remote office and it will ring and after X number of seconds the remote office will answer or just start ringing its phone. The problem you will have is if you answer the call using the analogue telephone you wont have the functionality to be able to transfer the call over trixbox.
You could have a trixbox server at each site. You dont say how big the external offices are or how many lines they have so I dont know how practical this is. But if you did have server at the remote sites they coudl connect to the analogue lines and be programmed to dial local IP phones and then transfer the call to the main site if not answered. This will also give you the advantage that you can use the IAX2 protocol instead in trunking mode. This will give you the advantage that you will be able to increase the number of simultaneous call from 3 to above 12.

For fax you cant really send the data across IP networks unless you use the T.38 protocol. Normally the easiest way is to either have a dedicated analogue line just for a fax machine or you can have a separate analogue phone interface on trixbox and get it to recognise fax tones on incoming calls and redirect them to the analogue fax extension. It would have to be an analogue phone interface in trixbox (like the pstn interface) and not an ATA adapter.

Personally I would use straight Asterisk and not trixbox. Sometimes these custom things in trixbox like redirecting offsite if not answered are not supported in the web interface and you have to configure vial the command line which immediatly gets much much more complex that if you used asterisk to start with. You do need more linux experience to install asterisk though.

You need a good quality of service for the VoIP connection. Typically the best way to go is to dedicate an internet connection just for VoIP which often means you have the VoIP dedicated connection in addition to your general connection which is used for email and web surfing.

There were quite a few questions there and hopefully I have helped and also pointed you towards various other things you need to think about.

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IP Telephony

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