Iphone 6 loses signal and does not come back

I have Iphone 6, 64 GB unlocked / on Software 8.3

It loses signal and does not come back although the data connection keeps working normally.

I discover that people try to contact me but I am suddenly unreachable while other phones on the same coverage area under the same operator are working very fine.

I tried to reset network settings / restore the whole phone fro earlier backup but it didnot help.

The remedy i use each time is to restart my phone to get connected back to the mobile network.

Please advise.