The base unit and headset are synchronized together when you bought them. So if you want to use a replacement headset with the base unit (or if you lose your connection to the base unit) you will need to follow these steps to synchronize the headset to the base:
Ensure the headset is in the base's cradle.
Press the volume control buttons on the back of the base until the front of the base flashes red.
Press the mute switch on top of the headset until the green light on the headset turns on.
Unplug the power cord for five seconds. After the five seconds are over, reconnect the power cord.
When the flashing red light becomes solid, synchronization is complete.