Treasure hunters are given a phone number that they must call. They must then find the phone whilst they can hear it ringing.
Make sure that the phone is hidden in a place where it can be heard when it is ringing.
Some thought must be given to what the hunters do when they locate the phone. To make the clue harder the phone could be locked with a PIN that the hunters have to find. To make the next step clearer, if possible remove all previous text messages so that the only message in the inbox is the next clue.