On being given the phone the Captain's decision to make seems to me to be; is this thing a suspect IED or is it lost property? Now there will be many mobile phones on the aircraft all of which have been security screened and are accompanied by their owners so we're happy with them. This phone is unaccompanied but that should make little difference in this era of suicide bombers? Therefore treating it as lost property seems reasonable to me. If it had been discovered before push back it would undoubtedly have been treated as lost property, not sure how a delay in its discovery makes it an IED but there may be facts we don't know.
However, we could also make the other decision and treat it as a suspect IED. No procedure I know says to return to stand with a suspect IED on board, hand it to the ramp agent who will then carry it through the terminal and hand it to lost property. Nor do I know of a procedure that says chuck the IED out of the DV window either to a waiting ops person or onto the tarmac. Both of those procedures seem to be the "we all know its not really a bomb but we're slightly covering our arses" procedures.
So, decide, IED or lost property. If lost property then treat as if its a lost paperback book or similar and carry out the relevant company procedure. If IED, treat it as if its a stick of dynamite wired to some sort of detonator and carry out the company procedure for that situation. The cobbled together not really sure but bit of both procedure is how we end up in this ridiculous situation.