I would start by checking the CFBNA settings... if, for example, you have extension 200, and they have another virtual extension of 201, and then voicemail, and they have it forwarded from 200 ==> 201 ==> voicemail, you would have twice as many rings before going to voicemail.
If you set CF Busy to 201, and CFNA to VM, then it would be OK....
Just my $0.02
"In order to start solving a problem, one must first identify its owner." --Me
--Greg