To prevent the phone ringing (from incoming calls), you can select Do Not Disturb.

To enable Do Not Disturb:

Press 
A flashing icon and text on the graphic display indicate that Do Not Disturb is enabled.

Calls received while Do Not Disturb is enabled are logged in the Missed Calls list unless configured to be forwarded to another party by your system administrator. 

To disable Do Not Disturb:

Press again .