Please follow the below step

1. Press the Page button (dot with 3 lines above it) on base for 2 seconds , the light next to the phone symbol on the base station will begin flashing to indicate pairing mode.

2. Press OK on phone prompt, then Select Register Handset.

3. Select Base1 on the Replace Base screen.

Searching bar will show progress. 

Make sure base still has the flashing pairing indicator during search.

4. When the base is found (will display a 11 digit serial # xxxxx-xxxxxx), press ok and you will be prompted for a PIN

5. Enter 0000 as the PIN and press OK.

When finished registering, you will see Handset 2 in the top right corner, and the extension will show when you press the Line softkey.