Switching from BT to Virgin

I've got Virgin as my ISP but I'm thinking of adding the phone as well and bypassing BT.

Problem is that I've just got my latest BT bill which asks for 3 months advance line rental. If I switch now, am I liable for this? Such is my level of mistrust with either organisation, I reckon I'll get conflicting (and probably wrong) information if I call them.

Anyone else tried this?

should get it back, I did when I moved to SKY.
Virgin broadband is ace. Can't get it where I am but have family with it.