within OBF, still faulty

I upgraded my contract and taken out a new contract both Lumia 535. On day 5 both phones were problematic, rebooting 3-5 times a day. I took both of them back to the store( with in OBF, BOTH WITH THE SAME ISSUE) the technicians declared that only one of the handsets were problematic and the other was not. They replaced the one handset which I picked up yesterday, the other, also picked up on yesterday( deemed to be working just fine), less than 8 hours the phone still reboots, ot happen thrice thus far and the technicians apparently could not find fault. I want these contracts cancelled. I want nothing to do with Cellc.please just take you phones and cancel the contracts. Its been a month and I literally only had the phone in my hands for 6days and still not satisfaction!