Are any of your parents neighbours on the internet? Where do they live and how far from the exchange do they live? This could affect how they connect to the internet.
There are several websites which give advice on connecting to the internet and the Which magazine have also produced articles.
Sorry I cannot be more specific as I am not an expert. I am with TalkTalk, but this includes my landline rental, as well as free calls at any time for a fixed price.
Be careful about signing up as some contracts are for eighteen months.