VOIP Project Help

I have been forced into a project that requires us to migrate our current phone system to an IP PBX system.  Currently we run off of analog lines and only have about 4 lines running into our office.  We are in the process of building a new building and will have a major increase in lines.  Here is our break down and would like to see what the best way of going about solution.  Currently looking at Fonality and Intuitive Voice for PBX system and SIP coming in.  We have a chamber of commerce that is going to build an business incubator for about 50 small (1 -3 employees) startup companies, Conference Room (1), Receptionist Area (1), Visitor Phone (1) and Chamber office phones (4).  Each office is going to have its own number (DID) plus extension for inter office calling.  When they call out the will use their own DID.  Not sure on Call Sessions as we don't know how many companies are coming in (maybe 20 to start).  We are going to have 1 - T1 to start out but will increase to a second T1 if necessary for bandwidth.  We will be using Polycom phones IP331, IP550, IP6000

Any advise will be helpful in getting this project started.
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jfaubiontxConnect With a Mentor Commented:
This question is really beyond the scope of what can be answered here. In fact an entire book could be written on this question. For a project of this size, I can not stress enough the importance of hiring a qualified consultant to ensure all aspects are covered. As an example due to the size of this project I would not be recommending using SIP connections to the PSTN unless a) you know what the expected call volume will be and b) you have enough bandwidth for the calls. Breaking this down though we need to know the number of concurrent calls that will be expected. Using typical rules of thumb, most businesses can utilize 20% trunking. Meaning that if you have 20 phones you would need 4 lines to service the calls. However if the business use involves is a call center, this trunking can increase to 100%-140% depending on acceptable wait times, average call length, and whether or not pre-dialing is being used. Until we know the number of trunks we need, we can not accurately determine the bandwidth needed. However lets consider least trunks case, most trunks case and an average scenario.
For the least trunks case assume 20 businesses each with 1 phones plus the 7 phones you have. Using 20% trunking means around 5 trunks. Using SIP with g.729b coding at 8k per call, even a DSL could be fine in this case. For the most trunks case assume 50 businesses each with 3 phones and heavy call volume. At 100% trunking we need 157 trunks. Using g.711u at 64k we need more than 10 Mbps of bandwidth which is at least 7 T1 lines for data. Now for an average case, assume 35 businesses each with 2 phones. With a mix of users lets say we need 50% trunking so we need 39 trunks. Using g.711u at 64k  we need 2.5 Mbps or 2 Data T1's. This all assumes that we have good quality to our SIP provider, we are not using these T1's to provide Internet access and we re not using fax machines.
As you can see there are a lot of factors that need to be considered and we haven't even discussed features, how to separate the businesses calling, billing for services, whether we are providing Internet services for them, how to lay out the network and many more topics.
If you are going to move away from normal PSTN lines and use VOIP lines only then you'll want to make sure you have a backup plan encase the T1 goes down.

Also if you imagine that there will be many calls internally then it might be wise to install a IP-PBX Locally at the site. However if this is not the case then Fonality would work too.

Also if you're thinking of not having a Local PBX then every call whether it be internal or external will use bandwidth so be sure to have enough spare to ensure good call quality.

Make sure that you either have a good QOS Policy installed on the Router or a separate internet connection for surfing otherwise its quite easy for one user to cause everyone's phone calls to break up.
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