Looking at the SIP trace, we receive a CANCEL message from 84.12.xxx.xxx so it appears that your phone system is clearing down the call before the voicemail of the mobile is kicking in.
There is media in both directions for 23 seconds (14:17:39 to 14:18:02) and then your PBX stops transmitting any media, the CANCEL then comes from your side 1 second later.
It would be necessary to investigate from your side why this CANCEL is being initiated from the PBX to further diagnose the problem.
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