We have been experiencing a small intermittent issue with things that I just can’t figure out.
When our receptionist is busy and cannot answer another line; sometimes (only sometimes so lets say three times a week), instead of going to the FWD No ANSR line, it goes to a cell phone that we only have in the system through the night ringing service (which isn’t on at the time these calls get diverted). Why would this be happening and is there a way I can prevent it from happening more?
I can't see anything at all in the BCM or the Callpilot that would be making this odd transfer.

what is foward on busy programmed to do

Nothing; it is blank so that typically when she is on the line, it won't go to voicemail but will still ring and get FWD NO ANSR after a certain number of rings.

does said line app/ring anywhere else?
easiest way to this is make the line private to 1 set log out of programming then login and make public then re-assign to the sets that should have it

Also check the forwarding of any other phone that might ring. If two phones ring the forwarding usually follows the lowest extension number.