Hi Danny,
You mentioned that you hear a busy signal while dialing your cell phone number. Do you get this busy signal before you finish dialing the number? Or, are you able to finish dialing the cell number completely?
You can also check your OBI's call history under Status -> Call History to see what it has to say about these unsuccessful outgoing calls to your cell.
If your problem is primarily intermittent (with complete success at times), then perhaps it is another issue such as network, lines, or firewalls. I assume your cell phone has its own voicemail service that is properly functioning right now. Perhaps you could let us know more information regarding your setup.
pc44