Debugging VoLTE issue: BM818 (Librem 5 Modem), MM 1.18.6

[Reinhard Speyerer]

On Wed, Jun 22, 2022 at 09:48:00PM -0400, Chris Talbot wrote:
> Hello!
> 
> I am trying to debug a VoLTE issue with the BM818. I am using T-Mobile USA with VoLTE enabled on the BM818, with a Librem 5, PureOS Byzantium 
> (based on Debian Bullseye).
> 
> With certain numbers, I have seen that the call terminates right after the call is picked up. However, With the same hardware/software
> set up with an AT&T MVNO (AweSIM, Purism's MVNO), I have no trouble with the exact same number. Additionally, Calling the number with the
> Pinephone Pro, VoLTE on T-Mobile USA (EG-25, MM 1.18) did not have this issue either.
> 
> For one number, I found that the `1-800` number had this issue (it was my bank), but calling the same number to it's local area code 
> (but the same bank call system) allowed the call to complete just fine. 
> 
> I am including the log below, but the log seems to show that the other side just terminated the call. What would be a good way to help debug/solve
> this issue?
> 

Hi Chris,

looks like the network terminated the call due to a SDP-related problem
- perhaps even before the call was connected - as TLV 0x2e contains
"SDP not supported".

It would help if T-Mobile could provide you the corresponding SIP logs from
their IMS platform for a VoLTE call with a Librem 5 phone and a Pinephone
when calling that 1-800 number for comparison purposes.

Checking in advance that your BM818 runs the latest firmware would help too.

Regards,
Reinhard