Hello Patrick,
You can send the DTMF during the call, there is not a problem doing that. Try this and let me know if it is succeeding or not. If not, I will adapt the dialplan to wait for the code and then initiate a local call to the door phone.
I can add some authentication to check if the caller id is an AMPUSER by adding few lines to the dialplan.
Thank you,
Daniel Friedman
Trixton LTD.
Tel: +41.79.868.7050
Email: dani@3xton.com