Welcome to the site.
Ok the precedure is for a ntl D505 phone so not sure if the registering of the phone is similar to that of the D5100, however both these phones look alike except, the D505 doesn't have the answer machine facility.
* Press the
Menu Button until
REGISTER is displayed.
* Press and hold the
Page Button on the base unit for 5 seconds until you hear two beeps.
Press the OK button on the handset and BS 1 2 3 4 will be displayed. Enter the number of the base unit (1 - 4) and the handset will display PIN.
Enter the PIN number (Default 0000)
* Press OK Button
* SEARCH will be displayed as the handset searches for the base unit. Once connection has been made, SEARCH will disappear and the number of the handset unit will appear on the right side of the display.
Hope this helps.