Another quick and dirty SIP interop post.
A while back I was tasked to interface a FreeSWITCH server and a Cisco Unified Communications Manager system. Once the SIP trunk was configured on the Call Manager/CUCM side they sent an INVITE over. It didn't have an SDP.
It appeared that we needed to enable 3pcc (third party call control) in FreeSWITCH. No problem. I enabled 3pcc and interop continued.
Problems arose, however, when we needed to send the Cisco ringback. Whether it be a 180 or 183 (with or without SDP for either) this was going to be tough because with 3pcc enabled the dialog looked like so:
INVITE (without SDP) <--
100 Trying -->
200 OK (with SDP) -->
ACK (with SDP) <--
So... There was no opportunity to signal progress as long as we 200 OKd the call almost immediately. Sure I probably could generate some ringback after the 200 but that would just be wrong!
As I like to say, the internet to the rescue. Not having much experience with CUCM I thought I'd ask on VoiceOps. Within a few minutes a very nice gentlemen by the name of Mark Holloway mentioned "Media Termination Point Required" as a CUCM configuration option. These were the magic words. After some research it turned out that was the configuration option I needed*. Thanks Mark!
Once "Media Termination Point Required" was enabled on the Cisco side I disabled 3pcc in FreeSWITCH and all was good. Users even get ringback now!
I also brought the issue up on the FreeSWITCH-Users mailing list and found out this has been bothering people for some time. MC from FreeSWITCH was even nice enough to start a wiki page for me to document all of this there.
Sometimes with SIP it's all about the SIMPLE achievements ;).
* That research also brought up another possibility: enabling PRACK/100rel on the CallManager side instead of "MTP Required". Of course the trouble with PRACK is there are a lot of SIP implementations (Asterisk) that don't support it. FreeSWITCH does but can crash. Many SIP implementations don't support the default CUCM configuration (INVITE w/o SDP). I was looking for the most canonical, compatible configuration possible.