You don't need to worry about your privacy when contacting agencies. A list of client contact numbers is never retained by most agencies. Because all recent calls and text messages are deleted after each shift and all incoming calls are screened such that, calls with no caller ID or private can't go through, or are required to announce a name of the caller before it goes through, or its sent directly to voicemail for the caller to leave a message and contact number if the caller was actually a serious enquirer.
However, most of the time all those extra precautions are not necessarily for the agencies who book calls on behalf of their girls and we do the screening manually, unless otherwise the girls will be answering the calls for themselves, if that's the case then the client should have every right to worry about the privacy of the contact number as same with contacting an independent. But not with agencies as we screen both clients upon booking and girls also upon hiring.
With the case of blocked numbers. Whenever booking, we usually just ask callers on blocked numbers to call back with a real number. Only blacklisted numbers are retained in order to block those psycho callers from ever calling back again, and callers from Apps can also be blocked. Those can be very disturbing especially the bp callers.