Surely you get access to a work phone. The ones on desks that are plugged into the little wall socket are the best, they use wires and so aren't subject to the variance in service caused by mobile signals. You will find the best quality desk phones are installed in open plan offices and the posh meeting rooms used by Directors and more senior staff.
Prossies are usually deaf, it's all the rattling they get whilst being repeatedly fucked. So remember, before you make that call, clear your throat with a loud cough or three, dial the number and announce at full volume and in your best 1950's BBC voice......"Hello my dear, how much for a nice hard fuck".........