IP SIP Door Phone/Doorbell (or analog coverted)

does anyone know of a good in expensive doorphone/bell that is compatable with freepbx?

I would like to setup to ring on ring group.

I was also looking at the option of somthing like the Panasonic KX-T30865-B then adding an ATA converter of some sort. has anyone done this?

http://www.akuvox.com/productShow.php?n_id=82

as a doorphone.