Please check the following:
- 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, press and hold 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.