Hi Viv. As it's a long time ago now, you may longer need the answer, but for anyone else with the same problem.
I just got one of these phones today and did the same thing - set the date and time and the ringer went to mute. This had me stumped for a bit too. I eventually figured out what I did.
If you hold the up arrow key down (the one with the 'x') it comes up with an option to set a time. I initially thought I was setting the time here (the time and date is actually set via the 'MENU' button). What it does in fact is set a countdown timer during which the ringer is muted (presumably for periods when you do not want to be disturbed, e.g. sleep).
If you reset this timer to 0:00, then the ringer will be unmuted.
Andy, September 2014
