I have had many clients tell me that if I ever need a referral, they would be happy to act as one. I usually ask them if it's ok to give out their phone number.

I also have given out the names of Realtors that have used me and said it was OK. I give out their office number.

In any case, I always ask if it's OK to give their number out.

It doesn't happen very often, same as when I'm asked for my license number or proof of insurance (from builder or listing agent).
JF