On no reply
If "Call forwarding on no reply" is configured on the phone, an incoming call that is not taken within the set signalling time is forwarded to the entered call forwarding destination.
Configuration
Web interface: 
Telephony > Call forwarding > On no reply
*Details about the settings are in Call forwarding
Function keys > Choose a key > Function: Call forwarding on no reply
*Details about the settings are in Function keys
Phone: 
Settings > Telephony > Call forwarding > On no reply
*Details about the settings are in Telephony
Settings > Function keys > Choose a key > Function: Call forwarding on no reply
*Details about the settings are in Function keys
Provisioning: 
*Details about provisioning this function in the Auerswald DokuWiki at Setting and Key