Hi Ma,

You will need to pre-build the relationship between ephones and DN's;

The SRST fallback support using Cisco Unified CME feature is able to interrogate phones to learn their MAC addresses and the extension-to-ephone relationships associated with each phone. This information is used to dynamically create and execute the Cisco Unified CME button command for each phone and automatically provision each phone with the extensions and features you want it to have.

You can optionally create a set of ephone-dns that are preconfigured with extension numbers and some features to provide service during fallback that is similar to the service that is provided during normal operation. Extensions that are not prebuilt are populated with extension numbers and features as they are "learned" by the Cisco Unified CME router in SRST mode at the time of fallback.

http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/admin/configuration/guide/cmesrst.html#wp1016614

Hope this helps!

Rob