This may be caused by a few things.
1. Check the configuration dial on the left hand side of the base to see if another setting works better. The recommended setting for most phones is 1.
2. Make sure the speak (transmit) level is not too high. First, try lowering the volume on the telephone. If you still hear distortion, adjust the volume of the headset. The master transmit controls are the ABCD switch settings on the bottom of the base. A is the lowest and D is the highest setting. You can fine tune this setting with the Plus (+) and minus (-) buttons on the back of the base that are above and below the picture of the microphone.
3. Double check the battery has a full charge.
4. Make sure the headset has been paired to the base.