Tel: If your broadband supplier is providing you with a modem - hopefully you signed with one who provides that free- the adaptor should come with the modem so you won't need to buy one.
Jaggythistle: Doesn't sound right to me, are you sure all your connections are right? One of the joys of broadband is being able to take calls without it affecting broadband connection and vice versa. I only had the sort of problem you are describing on dial up.