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Calls over SIP Trunk ?

Hi all experts.

I have a very simple scenario. I have one 3845 router and CUCM 7.x. I have taken SIP trunk from our local ITSP. The calls will be coming using g.711 codec. Now i have 20 branches, all of them using g.729 over WAN and our HO (where 3845 with sip trunk from ITSP is placed) is using g.711 codec on LAN. As you can see, so far its very simple. I want to ask, that if a call comes from outside (over SIP trunk) to any branch extension(which is using g.729), is transcoding required ? since call is coming from g.711 and going for g.729 extension !!

Kindly guide me

Cisco Employee

Re: Calls over SIP Trunk ?

- From the CallManager perspective, you need to consider the region settings between the phone and the SIP trunk.

- Region settings determine the max amount of codec bandwidth that can be used for a call flow between the regions in question

- If your SIP provider does only g711, but the region config setting between the phone and the SIP trunk is g729, then a transcoder is needed

- Call flow would be Phone --- g729 --- transcoder --- g711 --- SIP trunk (Make sure that the MRGL of the SIP trunk has access to a transcoding resource)

- If 'MTP Required' is checked on the SIP trunk (or) MTP gets invoked for DTMF, then that would bring in an additional variable into the picture

- Phone ---- MTP --- SIP trunk. If the MTP invoked is a transcoder, then call flow would be simple : Phone --- g729 --- MTP --- g711 --- SIP trunk

- Sriram

Cisco Employee

Re: Calls over SIP Trunk ?

If your SIP propvider also offers g729 in the invite, then it will just connect the call as g729 straight through all the way from the phone to the provider.

If your trunk *only* does 711, and you're doing g729 only to your phones, then you'd have a codec mismatch at CUCM and would need a transcoder registered to CUCM.  Anytime you have to change codecs in the middle of the call, you need a transcoder at the point where the codec mismatch occurs.

Also, note that CUE doesn't support g729.  You need to do g711 to CUE, so if you force g729 on your phones you need a transcoder for any call from those phones to VM.

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