Page 30 of manual at the link shown suggests a way to silence the Jitterbug.
Quoting from Page 30:
To adjust the ringer volume:
Be sure the phone is open and on.
on the outside of the phone below the
display window. Press each time you want
to advance to the next choice.
These choices will appear on the screen:
- Ringer Volume Ring Max
- Ringer Volume High
- Ringer Volume Med
- Ringer Volume Low
- Ringer Volume Silent
When you're satisfied with the setting,
release the volume button.
Your Jitterbug may ring and vibrate
to alert you to incoming calls. The
ringer and keypad can be silenced
by pressing the volume button
appears on the screen. Your
Jitterbug will now only vibrate to alert you
to incoming calls (This can come in handy
at a movie, a library, or any other location
where a ringer may be a distraction.