Call #2 is when you arrive at her address to get the apartment # and be buzzed in.
It's a security measure.

(Yop, I"m an oldster). the two-call system was set up to confirm that a client was actually at the location where a provider could look out a window and see the client.  

 
This involved pay-phones, and the provider would direct a potential client to a pay-phone that she could see outside her window, (the second call) to see if they were really there or if they were just pranking them.   If the provider could see the person from her window, they would then give them the address and apartment number and they would come on up.

 
Something akin to this is used today using cell phones.    Recently a gal asked me to snap a photo of her hotel on my cell phone and then send her the pic as a way of ensuring that I was actually there.