First is to check the Added PBX option on Durafon1X. There are 2 ways to verify the PBX option:
#1 Press Menu, use the UP and Down button and look for PBX option
#2 Press Menu + 5 + 0
To use PBX option, just press the flash key, wait for the dial tone, then the handset will ask to you to redial.
Press the flash key again
Then press the softkey for "OPTION". Select the saved PBX feature code. (use the number that corresponds to the PBX option to select)