One way audio via Sip trunk

Audio issues are nasty, especially when they are sporadic.
In such scenarios, it is important to isolate if we are facing issues on inbound calls or outbound calls and collect detailed CM traces with Sip messages enabled.

In the CM traces, all we have to look is the session description protocol (SDP) information.
Through CM traces, we can understand if we are sending early offer or delayed offer.
(In early offer, we send SDP information alongwith Invite message, the opposite happens in delayed offer.)
Below is the snippet of such a scenario-

Content-Type: application/sdp

v=0
o=CiscoSystemsCCM-SIP 2000 1 IN IP4 <ip address>
s=SIP Call
c=IN IP4 <ip address>
t=0 0
m=audio 25494 RTP/AVP 0 101
a=rtpmap:0 PCMU/8000
a=ptime:20
*a=inactive*
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15

Another sample of SDP in such a scenario-

v=0
o=CiscoSystemsCCM-SIP 2000 1 IN IP4 <ip address>
s=SIP Call
c=IN IP4 <ip address>
t=0 0
m=audio 18800 RTP/AVP 8 101
a=rtpmap:8 PCMA/8000
a=ptime:20
a=sendonly
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15



The part in bold is SDP information.
The part in red is where we see the problem. And who is causing it? The party which is sending this SDP information.
If it is CUCM, it calls for a caveat , what else could it be! ;)


Intermittent MTP leak in certain SIP call scenarios causes call failures

Symptom:
MTP resources may be intermittently leaked, ultimately resulting in failure of
SIP calls that require MTP resources. From RTMT, available MTP resources reach
0 and MTP allocation failure counts go up for each call requiring an MTP.
The SDP portion of the initial INVITE will incorrectly contain "a=inactive".

Conditions:
This issue can only happen in an outgoing initial SIP call setup where MTP is
required. In this case, the outgoing SIP INVITE message will contain an SDP
offer. The issue may occur in the following scenarios:
- Outgoing SIP trunk calls with "Media Termination Point Required" checked on
the SIP trunk.
- Calls between IPv6-only endpoints and IPv4-only endpoints.

This problem does NOT occur in the following scenarios:
- outgoing delayed-offer SIP trunk calls, even if an MTP is required.
- incoming SIP trunk calls.
- mid-call incoming or outgoing INVITE messages.

Workaround:
1. Uncheck "Media Termination Point Required" on the SIP Trunk configuration if
possible.
2. If Early Offer is required, configure Early Offer but leave "Media
Termination Point Required" unchecked.
3. For IPv6 deployment, use dual-stack rather than IPv6-only endpoints.