A secret
phone, typically a cheap prepaid one, used in a clandestine manner away from one's SO for the sole purpose of arranging booty calls from someone other than the SO. The purpose of the
bone phone is to avoid detection from the SO looking at your phone
bill.
Dave: Is that Lloyd's new
phone? What a loser -- why didn't he get an iPhone instead of that cheap piece of shit?
Robert: He has an iPhone...that's just his
bone phone. I think he's talking to your mom