I am with Virgin but thinking of switching to BT, wasn't very happy when Virgin put mine up to 17.99 with no warning plus we found them difficult to get hold of - and getting my MAC code ready to change took three attempts! Bt give you 3 months for 7.99 and then 15.99 after. I have my line and free wkend/eve calls with them already.
You should check which are available in your area, not all  of them are.