I would tell BIG that they are no longer allowed to call you and that if they want to discuss the matter further, they need to contact you either via snail mail or email address. Keep telling them that and refuse to discuss the matter further- do not continue to talk with them on the phone. Hang up on them once you realize it's them, or don't answer the phone at all.
If they want their money bad enough, they *WILL* contact you by email and/or by the terms you told them. They are going to be difficult until you start being firm and adamant with them.
LOL...this company is well known for their harassment. We should start refering to them as "The Notorious B.I.G.".