InterCom
The InterCom function enables an audio connection to a system telephone from one or more telephones without someone having to actively receive a call (for example, in a medical practice).
Process 
To set up InterCom on a phone number:
1. Dial the programming sequence for InterCom on an internal telephone.
The following rules/restrictions apply:
The destination can be either a single user or a group (several users).
2. The PBX system sends the SIP alert info for Auto Answer to the user's telephones.
The following rules/restrictions apply:
Only users who have permission to use InterCom are taken into account.
A SIP alert info is only sent to telephones that can evaluate it.
3. A free telephone (or more than one) automatically receives the InterCom call and enables the audio connection in one direction.
4. The connection is terminated under the following circumstances:
After 5 minutes.
When one side hangs up.
*Description of operation under InterCom
Configuration 
Switching on the SIP alert info for Auto Answer for telephones that can, and are to, receive InterCom calls.
Telephones > Assignment >
Switching on InterCom permission for the user whose telephones are to receive InterCom messages.
User > Phone numbers > > Configure