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

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?

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)