The problem here is : it is the PRI/BRI "listening" for DTMF, it does not know if it is a user pressing keys or a wav file played. So the idea of a wav file is nice but it will not work.
Westi's idea looks good but then you first must disconnect the doorphone, wait for it to ge free ( takes some time on many models ) and then dial the doorphone to open the door which still will not work as you ae a twinned user.
The only option is to dial a DDI number routing to the doorphone, then it is not a twinned call and you can send any digit as you want.
As a alternative you could use the internal relais contacts to open the door. You get called, diall **, get dialtone, dial the shortcode for activating relais one and dial ** again to do back to the door phone and disconnect.