800 Service review: Ongoing calls

Continuously recieve calls from this number. Never anyone on the other end. I have called back and told them not to call my number to no avail! Last call they made to me, 04/02/08, connected me to thier voicemail like I had called them. I get 4-5 calls a week from this number for the last month and a half to two months.
HELP! MAKE THEM STOP!