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