iphone 4s 16gb on tmobile no service still under warranty ?

iphone 4s 16gb on tmobile but it can be unlocked witha a gevey sim and can be used abroad then. bought it a month ago it was working all fine before but now wont let me make calls.shows tmobile with full signal but when u make a call it says call ended and call failed.all the other funtions works properly as a normal phone apart from calling and texting.im located in hounslow / west london im not willing to travel anywhere call me on 07565367587 no texts no offers 325is my last price as i bought it for £420 plz do not text me with ur offers i ll not reply so plz save ur text messages.and as long as the add is here that means the phone is still available.when the phone is sold i ll delete the add aswell.and its just a phone on its own no accesories or box. phone is in excellent condition