Focus cmax won't unlock?

Ford Focus c max zetec tdi 2004. Problem couldn't unlock the car in a carpark, called AA out he tried the door with key wouldn't open but the boot did, climbed through put key in ignition turned to position 2 (dashboard lights on) pressed unlock doors opened. He said he thought maybe a phone mast blocked the signal in the fob. However it happend again the following day. Any ideas?