ICE - missing srflx candidates in answer
Server reflexive candidates are not added in call answer if UPNP is disabled. This can cause ICE media negotiation failure if the peer is behind a NAT with TURN enabled, and UPNP disabled/unavailable.
Setup: Endpoint A TURN enabled UPNP disabled
Endpoint B TURN disabled UPNP disabled
A calls B. Results: failed (notice the absence of srflx cand in B's SDP answer) Expected: success (connection pair A:relay <-> B:prflx
ICE fails because the TURN server does not authorize connection from prflx candidates because they are unknown to it (they be authorized if the B add the address as a srflx canddiate).
Note that the call is successful if B calls A (because the srflx are included in the offer).