Putting a piece of metal close to the phone's antenna is never a good idea since it affects the reception and may force the phone to transmit with higher power to maintain a connection with the base station. If that piece of metal is magnetic, the performance is affected even more.
But there is no long term damage to the phone (except quicker draining of the battery leading to more frequent charging which may cause it to go bad sooner). As soon as the metal piece is removed from the phone's vicinity, the phone will be back to working normally.