Hi Experts.
I’m experiencing VoLTE drop calls, and the SIP code is 8003, even though the radio conditions are excellent.
Any idea what could be causing this?
Admin note: this post was updated with image below.
8003 as SIP Reason code?
Yes, correct.
But it is out of SIP standard.
I don’t know what this is, but SIP responses are defined under rfc 3261.
https://www.rfc-editor.org/rfc/rfc3261.html
Based on this explanation, it seems that voice packets (RTP/RTCP) are not transmitted for a certain period.
Are you testing this yourself? Does the call start with mutual audio, or does it begin as a mute call?
The call was perfectly fine initially, but then it dropped.
Are you certain that two-way audio was being heard throughout the call?
Yes, 100%.
Then the issue might be related to RTCP.
One of the clients may not support RTCP.
Do the calls always drop after the same duration, and does the result vary with different clients?
I would say about 70% of the calls drop after 2 minutes.
Try disabling RTCP and see if it resolves the issue.
One phone consistently behaves this way, regardless of the B number.
Any idea how?
Can you share PCAP?
I’m sorry, I can’t.
The SBC should have a configuration setting for this.
Let me reach out to Core guys, thanks!
This fact 
Support this thesis 
It means there haven’t been any downlink RTP packets for the last 20 seconds.
Check that the UE’s transmit power for each device isn’t maxed out, like 23 dBm.
Also, ensure the RTP-RTCP timer is set to a reasonable value, such as 15-20 seconds, and not something too low, like 1 second.
Ensure that both UEs are free from RLF (Radio Link Failure) and RRC re-establishment, and that all three DRBs are quickly re-established after any RLF.
No RLFs.