I called this girl but she called back with a different number. I am guessing that her friend is calling me? Any thoiughts?

quite possibly you called a # and she called u back from her disposable cell number?

(answering service, booker, agency, co-op) might either recieve a forwarded call or be booked from one number but have a "work phone" on her.   A Favorite lady also transitioned from one number to another...  
I'd not be concerned about the phone number but would require any provider to be well reviewed preferably here on TER.

The number I use in my advertising is not even connected to a real phone.  It is a virtual messaging service only.  I call back from a real phone.  Many ladies who do not want 3am drunk-dialing lookie-loos will do something similar.