You can mention the lack of free texts and they might upgrade you and throw it in as a good will gesture (maybe), but most likely the lack of coverage/reception should be good enough of a reason to cancel, especially if it has only been a few weeks since you got the phone.
My sister got a Blackberry from T-Mobile and I went in-store and explained the signal issue and they cancelled the contract, even though the contract was taken out in-store initially and you don't get the 7-14 day grace period like you do with online phone orders when you sign up at the shop.