I only give out my landline number but I get telemarketer calls on my cell phone and there are only 8 people on this planet that I have given my cell number to.
I don't think people realize telemarketers are NOT going off some list with your name and phone number. They know what prefixes that are in use in your area code and just start dialing. In the 213 area code they know 333 is an active prefix and their computer starts dialing at 415-333-0000 and keeps dialing all the way through 415-333-9999. They have no idea who the individual is they are calling.
I know from work, where we have I have to answer every call no matter what the caller ID says, that about half the calls come from phones where the ID is blocked. It comes up as "Anonymous" or "Wireless Provider" on our caller ID.
AND MOST IMPORTANTLY, don't EVER call a toll free number if you want to keep your number private. The call is free to you, the person you are calling is paying for it and because of that your number is revealed to them even if it is blocked. And that also exempts them.......and all their affiliates from the Do Not Call rules, because by calling them, you have established a business relationship with them.