Great telephone based prank. The victim can't figure out what's going on - Something is wrong with the phone. They can hear the callers, but the caller cannot hear them. (Alternately the joke can be played with the roles reversed with the victim not hearing the caller, but it seems more effective if the 'victim' can hear the caller).
Hello, Hello? HELLO!, Yes i'm here! Wait, I can hear you.... Hello! CLICK! Dial Tone.....
The joke requires the temporary modification of the telephone handset. The joke works with virtually any telephone that has a phone base and a separate handset that are connected together by a modular (snap in) connector. Handsets with pushbuttons on them will not work, nor will telephones that have the handset and base 'hardwired' with no modular connector.
All you need is:
1. 'victims' telephone
2. Some transparent tape
Step 1: Locate the Modular Connector on the Handset and Disconnect It.
The handset (the part you hold up to your head) is connected to the telephone base by a coiled cord with 4P4C connector (often called RJ9, see http://en.wikipedia.org/wiki/Modular_connector#4P4C). The cord and connector carries 4 wires; 2 for the speaker and 2 for the microphone. The joke involves selectively disconnecting either the microphone or the speaker in the handset.
Locate the connector on the bottom of the handset (part you hold to your head) and disconnect the cord. This is done by finding the tab sticking out of the connector and pressing it toward the cord while gently pulling the connector away from the handset. It should easily slide out of the hole in the handset. Do not force it.