This VAS is described in section Call pickup.
1. Add a new Call group (Telephony -> PBX -> Call groups -> add), and name it for ex. Managers
2. In user's SIP settings choose this group as a call group.
After this calls to this user will be linked with call group Managers, and each user which has this group configured as Pickup group, could pickup calls to this user.
3. In user's SIP settings add this group to Pickup groups.
Afterwards this user will be able to pickup calls which are linked with pickup group Managers.
By combining these 2 options (Call groups and Pickup groups) one can create complex access levels for call pickup feature.