here's the trouble customer is having company A has 2 hunting lines with fair point voicemail company B has forwarded there line to company A / when a person calls B it will forward and ring in on comp A lines but if both lines are busy it wont got to comp A voicemail instead you get a message ( if you have a mailbox on this service please enter it now) company A lines forward to off prem VM fine / Any Ideas ?

Contact the carrier; the CO might not support chain forwarding. It sounds like the voice mail system doesn't recognize calls originating from Company B as being a voice mail subscriber, so calls receive a system prompt instead. Same as if you have a voice mail forward path on a phone, but no mailbox.