Reference11r2:Concept Group Pickup across PBXs
Starting from version 11r2 group pickup can be performed across different PBX. The PBX uses internally dialog subscriptions to get the information about calls to all interested parties. A user can use this feature by using subscriptions to a group with partner keys. A group is identified by a prefix of '.' before the name (so if you have a group named groupname, then you would subscribe for the name .groupname). The user subscribing for the group dialog info, must be an active member of this group.
The partner key would then work like a pickup key (as you subscribe to a group).