How to use Intercom:
1. Pick up the handset and listen for a dial tone.
2. Dial your own telephone number.
3. Listen for a busy signal, then hang up.
4. All your extension phones will ring.
5. The ringing stops when someone picks up an extension.
6. Pick up the handset at your end and you are connected.
7. If no one responds, pick up the handset and hang up to cancel Intercom.