A new update, released today, adds additional functionality to Protel LTS.  For internal PBX calls - extension to extension - you now have the ability to override, or "spoof", the extension ID number that gets displayed on the receiving telephone.  This is useful if internal callers are calling a group extension number or SLA (Shared Line Appearance) extension number to reach an individual.  You can now have that group number display on other phones when making internal extension calls that originate from 1 or more users in that group, instead of the actual peer extension registration number.  This allows you to publish group numbers as extension numbers and gives you the ability to have additional extension users "share" or subscribe to the group to answer inbound calls.
To use this function, make sure your PBX has the latest update release.  If it does not update automatically on login, run the Update PBX command from the Maintenance menu.  After the update has been installed, run a Reload PBX command.  Under the SIP & Dialing tab for each extension, there is now an Extension Display Override field; this is where you will enter the desired group or SLA extension number.