You can select Do Not Disturb to prevent the phone ringing on incoming calls.
Enabling or Disabling
To enable Do Not Disturb:
Press Do Not Disturb on the right side of the phone.
A flashing icon and text on the graphic display indicate that Do Not Disturb is enabled.
(also a flashing "X" displays next to the line indicator on multiple lines.)
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 Do Not Disturb again.
If multiple lines are configured on your phone, Do Not Disturb can be set on a line-by-line basis if the feature was enabled by your system administrator. When Do Not Disturb is enabled for a particular line, a flashing “X” displays beside the line indicator (see illustration above).