Please check the followings.
- Active Mute: if a slashed musical tone icon appears on the screen, your phone is in the active mute state (i.e. no ringtones). To disable it, long-press the "#" key.
- Call alert setting: go to Menu > Settings > Tones > Call alert and make sure it is not "Off."
- Ring volume setting: go to Menu > Settings > Tones > Ring volume and make sure it is not "0." To increase the ringtone volume, you can also press the Volume Up key on the side of S0-20 from the main screen when you are not talking on the phone.