Are you using MGCP? If so, do you have the "call-manager fallback-mgcp" command? Do you have a POTS dial peer pointing to the PRI? Do you have one of the following depending on your version of IOS?
For Cisco IOS Software Release 12.3(13)T or earlier:
call application alternate DEFAULT
For Cisco IOS Software Release 12.3(14)T or later:
application
global
service alternate Default
Take a look at the following link.
http://www.cisco.com/en/US/tech/tk1077/technologies_configuration_example09186a008012ecc6.shtml
Hope this helps. If so, please rate the post.
Brandon