I've just read that you have to sign a new contract so if yours is say only half way through you then start again meaning it's longer until you can switch. Also there is a known problem with it stopping your called display although they will add it again if this happens.
We like BT and will probably stay with them but we are ex directory so we show as withheld. It's cut our unwanted calls a lot as has signing up with TPS. People mistakenly think that if they see a withheld number it means it's a spam call. This is not true at all as for instance if your doctor calls you the number is unusually withheld.
We still get one or two cold calls, but decided some time ago to let all our calls go to the answerphone, to save being bothered with unwanted calls. Guess what? When no message is left the other half IMMEDIATELY gets up and dials 1471 to see who called. I regularly remind him that if it was important a message would be left, and the reason for the answerphone is that we will not be disturbed! don't know why I bother.
I showed the BT link to DH. He had a look at it and said that it was not quite what we needed, as our international numbers for example are sometimes genuine ones. And ditto some of the other categories that come up such as external or suchlike.
So we have "invested" [£90] in something that was apparently on Dragons Den called trueCall CallBlocker.
It essentially screens calls before we receive them. If the number isnt on a set of numbers we give the machine that we say are ok, the person[if there is one there] has to give their name. If no name is given[they are asked twice for it]the call and caller does not get through to us. Bliss! If the person does give a name, then the call gets put through and we have to press the star key to tell the machine we will accept it[after hearing the person give the name].
I think it is going to be just what we need, but as I said we have only had it two days so far.