Hi Patrick,
Once the door phone will call you and you will answer the call, just make a blind transfer to *66 (or any other dial pattern that will be configured). On the worst case (the door will not open), The other person will call you again.
I am surprised that this door phone does not react on DTMF signals like the majority of the intercoms.
Thank you,
Daniel Friedman
Trixton LTD.