bt synergy 4500 is not ringing when incoming call?

my phones of which there are three handsets have all stopped ringing when someone is trying to call. I have unplugged the base unit and re plugged in but to no avail. Please can some one help. It displays on the screen that there is someone calling but it is just not ringing

m.amin, May 2010

< and > = vol button up and down

Handset ringer volume
The handset has 6 ringer volume settings: Off and 1–5
where, 1=low and 5=high. The default setting is 5.
1. Press < or > until Handset Settings is displayed
then select the OK option.
2. Handset Ringer Volume is displayed, select OK option.
3. Press < or > to display either External Volume or
Internal Volume then select the OK option.
4. The current setting is highlighted. Press < or > to
highlight the volume you require.
5. Select the OK option and you will hear a confirmation
tone.
6. Press and hold to exit and return to standby

me, May 2010

Handset ringer volume
The handset has 6 ringer volume settings: Off and 1–5
where, 1=low and 5=high. The default setting is 5.
1. Press ^ or ^

HAY88, May 2010