The process is similar to the delivery of email: The caller uses the SIP host name to look up the destination host, first trying a SRV record and then "regular" DNS, just like an email client (MTA) looks up the MX record. (SRV
records are generalized MX records applicable to any network service, including, but not limited to, SIP and RTSP.) For example, when contacting
sip:bell@cs.columbia.edu,
the client finds a SRV record pointing to
erlang.cs.columbia.edu
as the SIP server for the domain cs.columbia.edu. As for email, a single domain name can resolve to multiple servers, allowing load sharing and redundancy.
The server located in this manner can then proxy or forward the call to another server.
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SIP FAQ : 
Why can a forking SIP proxy not be stateless?