The NET/Callback key provides access to the following functions depending on how it is operated: 

Operation  Function  Press  Open the Net carrier list  Press and hold (min. 1 second)  R-key function:  Insert a pause (e. g. between the prefix and the phone number, or when checking a mailbox) End connection to a mailbox (e. g. when performing a callback, setting up a conference or transferring a call)