Testing two phones both (Aastra's and Polycom's with PRACK support that cannot be disabled) behind a NAT registering to OpenSIPS with the same name. These tests were in support of PUA_BLA.
When an incoming call is made by an endpoint that also supports PRACK, (while both phones ring) only one of the two phones can be answered.
When an incoming call is made by an endpoint that does not support PRACK, both phones can be answered.
Tested using a SPA942 as the phone calling in (which allows PRACK(100rel) to be turned off) resolved the problem.
Tested with FreeSWITCH and it initially failed because (100rel was enabled by default). When I turned off 100rel the problem went away.
I believe the problem is caused when the PRACK is being sent to endpoints that have been parallel forked by OpenSIPS.
Also, if I only use one Aastra/Polycom (powering the other one down), the single phone is able to be answered all the time.
The following thread first caused me to look into OpenSIPS and PRACK a little closer:
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