This method of twinning twins your normal phone with another internal phone. Your incoming calls will ring on both phones. You can also make your calls from either extension.

A typical example for internal twinning would be a user who uses both a normal desk phone and also needs a mobile phone while moving around the building.

Your settings are applied to both phones. Similarly, message waiting indication and voicemail access from either phone will be to your mailbox.

If both phones are phones that store your contacts and call history on the system, the contacts and call history on each will be the same.