I believe the reason for the change (apart from change for the sake of change to keep the publishers on their toes) was that "back-call" was a term commonly used by commercial salesman, and the WTS at the time was trying to emphasize that their work was a non-commercial ministry.
This is what we were told back then as well. My husband came into the org in the 90's and he said the service meeting was just a sales meeting with a different name.