Phdn = phantom ext. , which phone system do you have a DK or CTX/CIX , if it is a DK then intercom #604 then hangup will clear, if it is a CTX/CIX do intercom #605, like phonemeister said you will need to press the Phdn then dial #604 or #605. What is the range of you ext. numbers ?
If you are unsure of the Phdn button go to another phone and dial intercom 590 then go back to the phone you want to clear and see which button the call came in on that will be the Phdn(590)