Yes, but if they say that they provide coverage for a particular postcode and they don't do that, even after being given a chance to put things right, they are breaching their side of the contract. Most large companies will pretend that they are above the law and will fob you off, ignore correspondence and threaten court action if you stop payments. Still, if you are willing to take copies of letters that you sent, with posting receipts, to court and put your case there, it could be a way out of the contract.
If the coverage maps suggest poor coverage for an area and you get poor reception, you obviously can't use that justify stopping payments.