First things first, make sure the headset has charged.
Pairing new mic to old base:
1.Press and hold the telephone (TEL) button on the base until the white light begins to flash (about 5 seconds), then let go
2.Press and hold the mute button on the new mic until the light on the mic blinks (about 5 seconds).
3.Place the new mic near the base (but not ON the base) and wait. When the TEL light on the base stops blinking and goes solid, they are registered!
**Please Note: Doing this will automatically deregister the old mic!**
If you meant to do Conference Mode with multiple mics on the same base instead, check out this article!