In the first ambulance service I worked for it was policy to pass the controller your initials when receiving a 999 call and the controller would then respond with theirs... idea being it was logged on the tape who said what.
One day I was working with a guy called John L****n and the controller was John H*****y; A job was passed to us and then the formalities:
Controller:"..Thank you John, My initials JH, and yours please?"
Crew: "Thanks John, JL."
Controller: "Roger Mr.L***n, Red Base out."
PW