Make your own telephone. Take two plastic mugs and a $20$ metre-long, strong thread. At the bottom of each mug, make two tiny holes. Pass the thread through one hole and secure it firmly with a knot. Similarly, pass the other end of the thread through the hole in the second mug and fasten firmly. Hold one mug and give the other to your friend. Ask him to move as far away from you as the thread will allow. The thread should be stretched tight between the two of you. Speak into the mug and let your friend hear you at the other end. At the end of the conversation, you can call 'over and out'.

Sound vibration travels along the length of the wire or thread and reach your friend. A loose wire is not effective in carrying sound as a taut wire.