iPhone 5 loosing network after 4 out of 5 calls!

I have an iPhone 5 that I had exchanged already 2 times.

After a call (4/5) my "Telus" goes to 1 bar only, than "Réseau indisponible" for 5 seconded and than comes back to full reception.

My provider tells me the batch of refurb phones must comes all with the same problem... I think it is an App I have installed that causes the problem or the settings I have.

I tried to reinstalled from scratch and it did the same but really less...

What could it be? Anyone experienced this so we could compare our settings or our apps?

I am tired of that! It also seems to cause my batterie to drain way too short (2-3 hours) to 0%.

Please help!!!