I have not tried a new sim-card. My existing sim-card work fine on the same phone when the phone has Android 11, and it also work fine on my XA2.
when receiving some calls on the 10iii with SFOS, there is just a beep and then the display shows an error of “no network connection” but the connection is there I can call back immediately, I think SFOS, even with VoLTE off, sometimes does not correctly drop the connection to 2G, and then fail. Together with the issues of not being able to send sms with VoLTE on, and the very often multiple receipt of the same sms, or an error saying “The MMS cannot be downloaded” even there is no MMS but an sms. All together it’s so unreliable and annoying that I gave up on my 10iii.
I have considered trying a different provider, but I just don’t want to change provider whenever there is a problem with SFOS. Some years ago I had to move away from 3 when they started using some domestic-roaming that did not work on the Xperia X I was using at the time.
My provider is Unotel, which is part of the TDC group and using the Yousee network, just like your provider Telmore, I assume their network setup should be the same.