Hi Chinnapat,
As per the design guide:
Considerations for Inbound Calls
Once a call reaches an Expressway-E, it is routed to the relevant Unified CM cluster through its corresponding Expressway-C. In deployments with multiple edges (multiple pairs of Expressway-C and Expressway-E), there are two methods to route inbound calls:
Call routing based on the calling location
In this scenario, a business-to-business call enters the corporate network through the edge that is nearest to the calling endpoint or user. Because the call enters the corporate network after traversing a minimal distance over the Internet, this approach focuses on using the shortest path to the point of entry as the means of providing the best quality experience. In this scenario, use Geo-DNS. Geo-DNS provides unique DNS responses by geographic regions based on the source IP address of the DNS query, and it is thus able to direct a SRV query to the specific edge servicing a particular region. In this way, the calling endpoint is typically directed to the edge nearest to its calling location.
Call routing based on the called location
It is possible to direct the inbound call to the edge that is nearest to the called endpoint. This approach has the benefit of reducing the video bandwidth consumed over the WAN, but it requires a more complex architecture that has some scaling constraints. For this reason, Cisco does not recommend implementing this architecture when more than two edges are deployed.
You can check the following for more details
http://www.cisco.com/c/dam/en/us/td/docs/solutions/PA/enterprise/collabpa.pdf
Manish
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