I also had a similar problem. For me, I first experienced it when I tried to call inside the new house we just bought. At some places, I had no reception at all, other places made it possible to get a call and hear somebody (not clear, but I could hear them), but to talk myself, I had to go outside.
Not really something to look forward to, not being able to call inside your own house 
So I contacted support. I already had some other unsolved problems before, and all these together were too much. I did send the phone back to FP two days ago, they will probably have to replace it 
Personally, I think it has something to do with the antenna and a weak connection, similar to these problems: Loss of mobile network connection on FP2