Frankly, I would want to hear their voice/telephone manner etc. before I even get to chatting about making an appointment, so how about including your number in your very first response email to their inquiry? Unless, of course, it's a sleazy, time-wasting email.. Why agree on a date and time, and then find out he sounds as rough as a bear's arse? Too difficult backing out at that stage. The process of booking you should not be laborious and time-consuming. Timewasters won't mind the endless to and fro because they want to be entertained, after all, but serious punters will be put off by the process being tedious and long-winded.