Vodafone had me doing all sorts of tests in my home, but the key question like yourself is, it used to work OK, why does it not work now when nothing has changed in my home? and in my case, at certain times of the day it was bad. They also blamed the length of line from my first socket outlet to my modem, and again I said it used to work OK why not now. They eventually told me the local exchange was the problem and is going to be updated. Your phone line is only one of a number of issues getting the broadband to your house, they will tell you your weak points but not their own.