One option would be short range radio frequency keyfob transmitters and small battery operated receiver modules that are activated by a specific code, like remote door locks for automobiles. The receiver module would fit into a phone case and could activate a buzzer or music module. They have about 30 meters range.
I've purchased the sets from Reynolds Electronics in the U.S. quite cheaply, but the site seems to be down at the moment: http://www.rentron.com/
Here some offerings from the U.K.
http://www.quasaruk.co.uk/acatalog/info_ZANDER_S4.html
http://www.maplin.co.uk/remote-control-receiver-decoder-module-and-keyfob-transmitters-35628