Can't make phone calls in the UK with my Pro1

Hardware Adaptation
#1

I’m in the UK for holiday at the moment. I’m not able to make phone calls, which is really annoying. Strange enough, I’m able to get a data connection and surf the internet. I can as well get and send SMS. But I’m not able to receve or make phone calls.

In the status bar a triangle is shown, which I don’t see if everything works like it should. The SIM Card is not the culprit. It works in another phone (Xperia X, with the same Version of Sailfish) . Whereas the SIM card from the other phone have the same problems, when inserted in my phone. I have contacted the provider, but he points my to the phone. I have never had such problems. before, when i was abroad. I tried already to switch the carrier and to switch to 2G Network. Both actions doesn’t help.

Any suggestions what else I can do?

#2

it sounds like there is only 4G coverage at your location. Making calls via 4G would require voice over lte, which is not implemented. However, I would not expect this in UK, operators there seemingly are relatively late in dismantling 2G/3G (in comparison to other countries)