Just wanted to make a short note regarding the behavior of Cisco GW about the SIP OPTIONS messages received from some SIP Telcos or other SIP endpoints.
Upon reception of OPTIONS message, the Cisco GW sends 200 OK response and by default will wait for 32 seconds in order to clear the OPTIONS transaction acknowledged. This behavior is clearly explained on Figure 8 of RFC 3261 where Timer J is referenced.
I have seen scenarios where Cisco GW receives OPTIONS messages withing less that 32 seconds, therefore GW does not reply to all of the received messages, and the other SIP end complains about this behavior; but actually Cisco GW is following the RFC correctly. If you are in this situation, please kindly get the other SIP side to increment the timer between the OPTIONS messages received to a value greater than 32 seconds, and the problem will go away.
Hope that this helps if anyone hit this situation.