Protocol will be slightly different whether you call to book a girl on the same day or your book a girl for a later date.
Same day bookings:
1) call to book the girl you fancy at whatever time is free. You'll be given the address to go to.
2) once at the address, call the same number just a few minutes before your appointment time. (Call too early and they'll tell you to call again closer to the time. Call on the dot and you might get hold up in 5-10 min queues. 30 wait are more according to my exp early mornings when the whole horny crowd tries to get the girl of their choice for the time that suits them). They'll tell you then which flat number to buzz in to be let in (or in cases like broken buzzer like currently at the water street building they let the girl know you're there for her to pick you up).
In advance booking
1) call to book the girl
2) on the day of the booking call again to say you're still having the meet. That's when they'll give you the address
3)once at the address, call the same number just a few minutes before your appointment time. They'll tell you then which flat number to buzz in to be let in.
Advices:
If you make a booking for a following day and some unexpected things happen or you change your mind DO NOT assume that not confirming on the morning will be enough for them to know you're no longer interested. Gotta be explicit from you. Else you'll end up on their blacklist and will no longer be allowed to book in advance, only on the day, until several bookings went OK and you'll finally be trusted again.
Do not hesitate to ask them on which floor the flat is. I have had instances where there's no logic like a number 173 on the second floor and not first as one would guess. (Daviemac 173 is an exemple, not the real number I was given which I cannot recollect precisely anyway)