The easiest way to disguise a voice on the telephone is to use a voice changer. A voice changer is a gadget that changes or distorts a person's voice.
Using a foreign language or accent is another effective way of disguising a voice on the telephone. Practicing full conversations is highly recommended to avoid breaking the accent. If someone, however, isn't very good at accents, he can hold his nose while speaking on the phone to produce a nasally sound.
Pretending to have cold or changing pitch can also be incredibly effectual in disguising one's voice on the phone. If nothing else works, an individual can try muffling the sound of his voice by putting a shirt, handkerchief or a towel on the receiver.