We had an inquiry today where someone wanted to add an intercom extension but could not get cabling and wanted a portable intercom extension as well. The intercom in question was a Fermax. We know that Fermax makes a phone interface and checked whether this could be added straight on the back on a video handset in this particular unit block. Answer was yes.
So with phone line adjacent it is possible to extend you intecom into your phone system and answer the door from any phone – and with most phone now wireless, you can take it outside with you to answer the door.
Next option would be to divert to your mobile and answer the door from your mobile wherever you are.