Using iPhone as an apartment callbox number?

I was just curious if there was a reason you couldn't use iPhone as the apartment call box number? I have it set up as is (I don't have a land line any longer) and the call from the box comes through, but pressing the button doesn't seem to give a sustainable tone to open the door. Has anyone else had any issues doing this? Is there a way to set up the iPhone to have a long keypad tone instead of a beep?