I ran to interesting case today. Customer recently implemented ACME SBC gateway in large Lync enterprise environment and SCOM immediately began raising alerts of type “Alert: The Mediation Server service has encountered a major call completion problem with the proxy peer"; with cause: “Calls to this Proxy peer failed 5 times with failure final responses.”
Lync SIP Trace of failed call showed “488 Invalid Media” which on first looked was strange, since not all calls were exhibiting this behavior. However, while examining the SDP, we’ve noticed the INVITE of failed call SBC offered had two “a=fmtp” attributes as shown.
A successful call includes one “a=fmtp” attribute only, as shown.
We could not establish a pattern at this moment (failed calls were observed from different Area Codes and caller ID and so, while the service provider should address the inconstancy, the immediate fix would be to create a rule on SBC to catch if “a=fmtp:101 0-16” is included in the provider’s offer and either remove it, or rewrite to “a=fmtp:101 0-15”. I personally would remove the extra “a=fmtp” and “a=ptime:20” lines, but both should work.