SIM card not 100% compatible with my phone??

Hello everyone,
I have a G54 5G with Android 15.
I've been experiencing something really strange since I switched to a Reglomobile plan (I migrated from Android 14 to 15 in the meantime, but the problem already existed on Android 14).
Specifically:
- I can call all my usual contacts without any issues, they can hear me and I can hear them.
- EXCEPT when I try to call my son, who by the way has also switched to a Reglomobile plan: when I call him, I can't hear him and he can't hear me.
- However, if he calls me, there's no problem, we can hear each other perfectly.
- For us to be able to hear each other when I call, I have to make the call via the Wi-Fi of my internet box, and if it's via my SIM card, it just doesn't work.
- I put my personal Reglomobile SIM card into my other work phone (iPhone), and it works fine, I can call him without any problems.
- If I put the work SIM card (Orange) in my personal mobile (G54), it works too.
I conclude that the combination of the Reglomobile SIM card and my G54 causes issues; however, it works for my son, who also has a Reglomobile plan but with a different phone, also a Motorola but not the same model.
What baffles me is that the problem seems to only occur when I call another Reglomobile subscriber, and that the issue is only on my side.
Do you have any explanation or feedback on a similar issue?
Thank you.