The earpiece may not be turned on. Press the call control button for approximately 2 seconds until you hear a tone or see the indicator glow blue (Discovery 925) or white (Discovery 975).
The earpiece may be out of range. Move the earpiece closer to the Bluetooth phone.
Your earpiece battery may be drained. Charge your battery.
The listening volume may be too low. Incremental tapping of the volume button will adjust the volume.
Make sure that the eartip fits properly.