CME as SRST is precisely what you would need:
When a phone registers for SRST service with a Cisco Unified CME router and the router discovers that the phone was configured with a specific extension number, the router searches for an existing prebuilt ephone-dn with that extension number and then assigns that ephone-dn number to the phone. If there is no prebuilt ephone-dn with that extension number, the Cisco Unified CME system automatically creates one. In this way, extensions without prebuilt configurations are automatically populated with extension numbers and features as the numbers and features are "learned" by the Cisco Unified CME router in SRST mode when the phone registers to the router after a WAN link fails.
http://www.cisco.com/en/US/partner/docs/voice_ip_comm/cucme/admin/configuration/guide/cmesrst.html
Just SRST does not allow you to pre-configure the ephone-dn config.
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HTH
java
if this helps, please rate