Hi Huy,

A Gatekeeper is not required in this design;

In a centralized call-processing model, the Cisco Unified Survivable Remote Site Telephony router facilitates automatic failover so that local calls and active calls from Cisco Unified IP phones to the public switched telephone network (PSTN) are maintained even if a WAN failure occurs. If the WAN link to a remote office fails and connection to the Cisco Unified CallManager for the domain is lost, the phones in that branch office automatically redirect to the Cisco Unified Survivable Remote Site Telephony router. The Cisco Unified Survivable Remote Site Telephony router automatically takes over and offers a rich set of telephony functions to help ensure business continuity with minimal impact. When the disrupted WAN link is restored, the phones automatically reregister with the original Cisco Unified CallManager-and again, no manual intervention is required. Cisco Unified Survivable Remote Site Telephony is accomplished through this integrated system with ***no additional hardware components.

From this good doc;

http://www.cisco.com/en/US/prod/collateral/voicesw/ps6788/vcallcon/ps2169/product_data_sheet0900aecd805e1e79.html

Hope this helps!

Rob