Problems interconecting CUCM 8.0 with 3rd party SIP trunk

[nachete1979]

Hi all,

I am trying to connect a CUCM 8.0 with a 3rd Party SIP Trunk. The  thing is when I start the call from a device on the other side of the SIP trunk the RTP is correct both ways. The issue is when I start the call from a Cisco SCCP phone behind the CUCM because RTP is only listened in one way (to the CUCM and not from it)

After having a look at the capture, we have seen the SDP in the set up of the call is different when the call is originated from the 3rd party from the one SDP when the call is originated from a Cisco phone. The problem is that when the call is originated from the 3rd party, the SDP shows that the RTP must be finished in the CUCM. On the other hand, when the call is originated from a Cisco phone, the SDP forces the RTP to be finished in the phone itself.

I know this could be solved if the 3rd party opens the whole ip space behind the CUCM, but that is impossible because of the same IPs are used inside the 3rd party network.

Is there a way to force the CUCM to be always the originator of the RTP when using an SCCP phone? Is MTP needed?

Thanks in advance

[Jonathan Schulenberg]

You can check the "MTP Required" checkbox on the SIP Trunk in CUCM. This will proxy RTP media through either IPVMSA or the IOS SW MTP depending on what you have configured in the MRGL. This is not a substitute for a session border controller though. Normally the way address hiding is accomplished is through an SBC such as CUBE.

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