CUCM - MGCP and FXS analog phone receives calls but fast busy on dial

This issue was due to previous configuration with H.323 dial peers. Using MGCP for any VOIP call would be routed to the correct voice port. My best guess is having the dial-peer that was not created from CUCM using the service MGCP would be blocked for routing. If your not getting inbound or outbound calls check to make sure CUCM is able to create new Dial-Peers upon configuration. See Example:

dial-peer voice 999010 pots
service mgcpapp
port 0/1/0

Reference: Cisco 2851

Not sure how many people are switching from H.323 to MGCP but hope this helps.