If you need to register a new headset or the accidentally de-registered headset to the headset base follow these steps:

  1. Make sure the battery is properly installed in the headset.
    The LED light flashes slowly in red and green to indicate that the headset is not registered.
  2. Place the headset in the headset base and allow it to charge for at least one hour before beginning registration.
  3. Remove the headset from the headset base.
  4. Press and hold the PAIR button for about four seconds, and then release the button.
  5. Return the headset to the headset base for registration. The LED light alternates quickly in red and green.

It takes about 60 seconds to complete the registration process. 

Once the headset is registered, The LED light turns red when the battery is low and being charged, or turns purple when the battery is well charged, or turns blue when the battery is fully charged.

If the registration fails, you hear the voice prompt "Not registered" when you press the ON/OFF button on the headset, and the ON/OFF button light on the headset alternates slowly in red and green.

See also https://service.snom.com/display/wiki/Registering+and+de-registering+DECT+accessory+devices