Withheld numbers are just nothing but trouble, you have no way of working out who's calling to confirm their appointment or not, if he gets lost last minute is early/late no way of chasing up, and the same guy can phone you umpteen times asking the same questions for 2 years - yes happened to me, denied it was him every time. Once let a guy I'd met a few times switch to a withheld and it was a nightmare (he's arrive early, phone me, leave a message to say he was early can he come now and of course I couldn't phone back, we spent a lot o time trying to catch each other on the phone at various points and he'd just greet me with "You know who it is").
Paying for sex with a woman who is under the age of 18 is illegal, and I'd not like to have anything to do with it - I can guess that anyone involved could be then under suspicion controlling her and I'd like to be a million miles away from anything like that.
You also need to be checking addresses before attending outcalls, either landline in directory to be spoken on, or copy of a recent utility bill and/or electoral roll check. Going out on the basis of a withheld number (or any mobile number) alone is running a much higher risk of being plain mucked around, or something odd happening and without proof he's at an particular address you may not be able to let your security buddy know where you are (bit late when you've got there and he says "Oh it's not number 1 it's number 3 come on in here" and your buddy thinks you are in number 1).