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