I always put 000-000-0000 as my number when I sign up at books and if I have to call I go call from a payphone, I'd never let them get my number.
When I first signed up with SBG, they put a hold my account a day after I opened it (after I had already made some wagers) telling me I needed to call and verify my account. During this call (from a payphone) they asked if I had a phone number where they could contact me (I put all zeros when I signed up per usual) and I told them no, I had heard too many horror stories about people getting cold called and I was not going to give them a phone number and if that was a problem then just shut down my account and send me my money back. They backed off and said "no, that's OK" at that point. I urge everyone else to do the same.
after reading all the crap on here about these clowns calling and bothering people constantly all day i also put in all zeroes on phone number! IF THEY GOT A PROBLEM F'EM