I think 4 rings before voicemail picks up is not enough, I found out on the MTS website you can change it on your phone so I changed mine to 5 rings. I just think 4 rings isn't enough time for the customer to answer their phone.
I was looking at this last night after work and it seems that most voice mail is defaulted at 4 rings (20 seconds). Some other companies have it default at 6 and their customers complain that callers hang up before the voice mail answers. As Amanda says its a no win situation.
I wonder if 20 seconds was due to research from the old Bell Labs who did all sorts of studies into human interaction with the phone system or if somebody at Octel (probably the largest of the suppliers of voice mail systems) thought that 4 was a good number of rings.