Check out this link for information on how to integrate UC500 with the Linksys ATA SPA8000 and the Cisco VG224 24 port FXS gateway (look under Voice Analog Gateways):

The default behavior for an FXS port (a port that you connect an analog phone or fax to)  on a cisco router is to act as a gateway port that is configured with h.323 in CallManager Express. 

We can make the FXS port act just like a SCCP controlled phone