The simplest way is to do what I did: perform a Google search for "unlock the keypad on the Eircom 8000" and look at the results.
Usefully, the very first result I got was to the manual, at
http://support.xgem.com/site/livret/DECT_UG_253254441A_Eircom8000_(D32T_V)_EN.pdf
I couldn't be bothered to read so just searched within the document for "unlock" and found the answer on page 20 or 21.
Acp, August 2017
