Have a look into internal twinning, I havent had a chance to use this myself but still this is what I think will solve the problem.
This is what the help file says about it:
Internal Twinning
Internal twinning is available on IP Office 3.1 and higher systems. It can be used to link two IP Office extensions to act as a single extension. Typically this would be used to link a users desk phone with some form of wireless extension such as a DECT or WiFi handset. Prior to IP Office 4.0 May 2007 maintenance release Internal twinning was not supported in North American locales. Internal twinning is an exclusive arrangement, only one phone may be twinned with another. When twinned one acts as the primary phone and the other as the secondary phone. With internal twinning in operation, calls to the user's primary phone are also presented to their twinned secondary phone. Other users cannot dial the secondary phone directly.
- If the primary or secondary phones have call appearance buttons, they are used for call alerting. If otherwise, call waiting tone is used, regardless of the users call waiting settings. In either case, the Maximum Number of Twinned Calls setting applies.
- Calls to and from the secondary phone are presented with the name and number settings of the primary.
- The twinning user can transfer calls between the primary and secondary phones.
- Calls will ring at the secondary if the primary is logged off or set to do not disturb.
- Logging off or setting do not disturb at the secondary only affects the secondary.
- User buttons set to monitor the status of the primary also reflect the status of the secondary.
- Depending on the secondary phone type, calls alerting at the secondary but then answered at the primary may still be logged in the secondary's call log. This occurs if the call log is a function of the phone rather than the IP Office system.
Hope this helps