Why wouldn't you just (1) get a local google voice number, (2) forward it to your cell, (3) ask who it is (yay security!), (4) press 9 (or not). I'm honestly not getting the value here.

I'm also totally not getting how the security aspect of this is being glossed over. To the suggestion that apartment owners/managers would be interested, why would they put up a security gate then install a system that allows it to be bypassed by anyone with your phone number?? I actually bet if you told your apartment manager he would demand you immediately disable it, not pay you to give it to your neighbors.