Well, after much annoyance my SIP server has understanding of the SDP it passes on. At least enough to understand "hold". This allows call transfers to work and not end up with one or both ends of the transferred call being "on hold". Not too bad when they are a SNOM with a hold button, but not good when it is a normal phone line calling in... Grrr.
And the gigaset did not like my accidentally sending Content-Type twice, but chose to totally ignore the message rather than actually complain... Fixed.
Anyway much progress and we now have all the features we had on the old system - some of which involves passing the call to the old system (simple IVR, voicemail, fax2email, IAX).
A voicemail app will be simple; an IVR not too hard; and fax2email may be challenging but I bet it is way better than * when I have finished.
The CDRs look like they are working, so ready to try making bills from them!
We now have the big step of switching most inbound calls in one go to the new server. Eeek. We'll try that soon...
All good fun.