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Help with VoIP Design

Planning on deploying VoIP for 120 IP phones. The build will have 2 CMs, 2 Unity Servers, 120 7940G and 2 4507R Switches. No WAN connection, all outgoing calls will go to PSTN. Will be using gateway module in 4507R for PSTN connectivity.

So my question is what would be a good PSTN connection. Meaning would I need PRI, Multiple PRIs or FXO ports work? I understand that using conventional TDM with a PRI I can make 24 calls. However I am confused on how this works, is there anywhere I can get information on this to clarify. How many PSTN Calls can be made at one time with PRI & FXO? Will using G729 Codec increase the number of PSTN calls?

2 REPLIES
New Member

Re: Help with VoIP Design

With a PRI line from your telco you will be able to make 24 calls either in our out. That’s 24 total. The Codec will not change this at all... In fact you would probably want to use the G711 codec since it is the highest quality and all your calls will be local. an FXO cars which usually has two FXO ports can make one call per FXO port.

For 120 people I would go with a PRI line from your telco. Then you can decide if you want DID numbers for your users or if you will have every one call into one number and then dial the extension... For 120 people 2 Unity servers would be overkill... I would scale it back to one Unity server, Are you going to use Unified Messaging? Defiantly stick with two callmanager servers it makes installing patches and upgrading soo much easier...

New Member

Re: Help with VoIP Design

A 'good' PSTN connection is heavily dependant upon your particular situation. For example, Business_A with 120 phones with 2 lines each may need 8 PRI becuase all lines are expected to be used simultaneously, where Business_b may need only 1 PRI because calling patterns have historically indicated that on average there are 10 simultaneous conversations taking place. You will have to figure this out yourself based on some kind of historical data.

One more note: a PRI will give you only 23 DSO's for active conversations, as 1 channel is dedicated to signaling, unless you have more than one where you can combine the signaling onto one d-channel.

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