When they are very busy, like on Mondays, the lines are all busy and you will get a busy signal.
A lot of the problem is people have given out MS's number so darn often here that people that are not members try to call and book other resorts and ask information (ignoring, of course, the instructions to call another number for information).
If you only have say 30 lines and they are all in use, you are going to get a busy signal.
I doubt it has anything to do with voice mail.