Fast busy after 7 rings (outgoing) - Asterisk - Debian Linux

Our Asterisk system is working great. One issue though, one particular client of ours has a voicemail system that kicks in around 8 or 9 rings (much longer than the norm). When calling from any office phone on the Asterisk system, phone rings 7 times, then goes to a fast busy tone. If the client is in their office and pick up the phone before the 7 rings, everything is fine, but we can never hit their voicemail. When calling from a cell phone, we get 8 or 9 rings and hit their voicemail just fine.

Any settings in Asterisk that can affect this?

do you have a timeout in your dialstring that’s cutting you off early ??