I hat a similar problem. I tried to start a call but the phone UI didn't start. After 10 seconds there was a sound of a failed connection. Then the phone UI was shown shortly to tell me it was not successful. BUT what the phone did in background was calling my contact without me knowing about it. Even if I would, I coultn't stop the call. The other side has to catch the call and hang up to end this situation.

I read on TJC about turning automatic search off and choose the provider manually in the settings. This seemed to help. You should try that, too. No failures since then.

Nevertheless, I like Sailfish OS. Maybe there are some bugs. I will go to TJC and post them. That's how it lives. I'm sure it's getting better.