Bought my youngest a new mobli for xmas. She used to have a cheap Nokia on T-mobile network, worked perfectly well wherever she went. She now has a Samsung but is still on the same network but she can't get a signal at home or at my mums house 20 miles away. My other kid is on T-mobile too and she has no trouble getting a signal (she has a Motorola). What is it that is stopping the Samsung from getting a signal? Any ideas?

Samsungs normally have pretty good sensitivity, get it replaced it is probably just a "deaf" example, it does happen.