The Doorbird can do this. I'd recommend using a pair of dry contacts inside the doorbird to ring a regular doorbell; then you'd use a dedicated iPad to answer the person at the gate and open the door.
This has the added advantage that someone not on site could answer the Doorbird on their phone and open the door... call it a gate in this case, for instance so a package could be left somewhere inside the gate. The Doorbird lets you look live, so you can tell when the person leaves the site, too.
A good answer is easier with a clear question giving the make and model of everything. "The biggest problem in communication is the illusion that it has taken place." -- G. “Bernie” Shaw
