A term usually used for a call girl who does more than just "escort" or "body rubs". It means they perform everything. Sex in all forms by a call girl is called "full service".
Caller: "Hi, I was interested in your ad."
Call Girl: "It's $150 for a hour."
Caller: "Is it just a body rub or full service?"
Call Girl: "No, it's just a body rub."
Caller: "Ok sorry, I was looking for full service."
This means he wants it all, and this means that she is not doing "full service" just body rubs.