A few days ago, I was on the phone for a long time with my brother, and every 6min30s, the phone call was stopped. We spoke for more than 30 minutes, so a lot of 6:30 call 
As my brother has his phone for a long time and never got this issue, I suspect the FP3, but I am not sure.
I was on battery economy mode, at less than 15% of battery. But my brother and I were both with a good network quality.
I do not have any proof, reproduced bug or anything, but I want to make this post in case someone else have this problem with FP3.