Hi Michael,

>We have a building with one of those phone keypads in the vestibule that calls a residential phone number. The answerer can then chat with the caller and then can buzz them in by pressing a key combination into their keypad.
>

So, the existing system uses an internal phone system? What make/model?

>We need to upgrade this system. I can figure out how to do the dialing part, but how do I watch for the tone combination from the tenant in order to activate the door lock - and what do I use to activate that door latch?
>

Your need a device to decode the DTMF tones - something the existing system does very nicely :>) Door opening is a matter of solenoid activation by the decoding device. I don't see this as much of a VFP project as a bolt in hardware solution. This kind of system is 'off the shelf'.

>Any references out there on this type of project?

Tons. http://www.google.com/search?hl=en&q=keyless+door+entry

>Michael