Your IP Office system administrator can configure you with a twinned phone. The IP Office system supports 2 types of twinning:
Internal Twinning - This type of twinning twins your Avaya Workplace Client with another internal extension. You can make and answer calls on either.
Mobile Twinning - This type of twinning twins your Avaya Workplace Client with an external telephone number. You can answer incoming calls on either.