Relaying this response to you:
There is not much we can do there. The most problems arise from VoLTE being active or not and this would be the details I would suggest to check or ask for. This is basically the only thing we have some influence on, the rest is handled by the modem firmware. The next step would involve to gather extensive debug logs from the modem for which we do not have the means, especially not on the L5 since these tools usually are Windows or Android based.
Doesn’t sound encouraging. I’ve tried with and without VoLTE, so perhaps I’m up the creek without a paddle now. Unless they find something specifically wrong with my phone. I’m really hoping that is the case otherwise I’ll not be able to use it as a daily driver, at least not until a new modem comes down the pipe.
Just a thought, if the tools are windows or android based, what would it take to mount this modem on some sort of android platform in order to drive test the modem itself?
EDIT: I have a couple Intel NUC’s I could add one of these (GORITE LTE Adapter LID for Intel NUC) to one of them and run Emteria OS (emteria.OS | Embedded Android) on it, to be able to load those tools (whatever they are). Just plug the NUC into the cigarette lighter and I can drive around and make test calls.
There’s an effort to document carriers and what worked/didn’t work for users: Cellular Providers · Wiki · Librem5 / Librem 5 Community Wiki · GitLab . I assume filling in more in that table would be welcome.
I myself have used the Librem 5 over the years without issues in Germany, Switzerland, Austria, Netherlands, Belgium, France, Luxembourg and Kosovo. Most of them via roaming but some of them with prepaid sims from those countries.