Why your VoLTE call drops at 183 Session Progress
A 183 with SDP is where preconditions, dedicated bearers and early media all collide. Four causes, in the order worth checking them.
A call that gets to 183 and then dies is one of the most common VoLTE faults, and one of the most misdiagnosed. The 183 means the terminating side has produced an SDP answer. Everything that fails after it fails for one of four reasons.
1. The dedicated bearer never came up
The 183 with SDP triggers the P-CSCF to send an AAR over Rx, asking the PCRF for the voice bearer. If that bearer is not created, preconditions never meet and the call stalls exactly here.
Check the Rx exchange first. An AAA with a failure result code tells you immediately. If the AAR was never sent, the problem is upstream in the P-CSCF's understanding of the SDP.
2. Preconditions are misaligned
If one side requires preconditions and the other does not support them, you get a stall rather than a clean failure. Look at the Require and Supported headers in the INVITE, and the a=curr and a=des lines in the SDP.
The failure mode is asymmetric: the side that requires them waits, and the side that does not support them thinks the call is fine.
3. The SDP answer has no usable codec
If the answer contains only codecs the originating side did not offer, or the direction attributes do not line up, media never flows even though signalling continues. Compare the m= lines and the payload types directly rather than trusting the summary in your analyser.
4. RTP is going somewhere else
Signalling can complete while media takes a path that is blocked. Check whether RTP is arriving at the address in the SDP, from the address you expect. A NAT or firewall between the media endpoints produces a call that sets up perfectly and has no audio, which then drops on a media timeout.
The order to check
Rx first, because it is a single request/response and rules out the most common cause in seconds. Then preconditions, then codecs, then the media path. Working in that order finds the fault faster than reading the ladder diagram top to bottom.