Here is a cool solution that makes your door buzzer smart: DoorBuzzing comes with a permanent phone number that works with your intercom. A computer will answer your door when you have a visitor. During business hours, your door is automatically opened. You can also ask for a personal pin code.
DoorBuzzing can call other folks if you are not available. You get an alert when a pin code has been used. It also stores entry logs for security. Plans start for as low as $5 a month.