Call Forwarding does not go into Voicemail

Hey all, we are a business that has call forwarding setup for after-hours and Business VoiceEdge.  We have rules setup so that if a call comes in to a hunt group and it is after normal hours of operation, the call get forwarded to a cell phone number not associated with Comcast.  The cell phone will ring properly with the forwarded call however it will never go into that phones voicemail unless someone hits ignore or decline call on that cell phone.  It will just keep ringing that phone in a loop it seems.  Is there a configuration that controls this or something that we may be missing on the account?  We can't figure out why the voicemail won't pick up the forwarded call.  If we call the phone number of the cell phone directly, the voicemail will pickup after about 5 rings as normal so it is only acting like this on the forwarded calls. 

Any help appreciated.