Normally what happens with DHCP is that a broadcast for an IP address is sent out and a DHCP server responds. Once this DHCP and Client confirm connectivity the DHCP server checks there isn't a static IP for the MAC address and that there is a IP free in the range. Then it grants the phone with a IP address.

Here is some more information on the subject.
http://www.cisco.com/en/US/docs/security/asa/asa72/configuration/guide/dhcp.html

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Hi Sandeep, I would like to add few more information as below:

When the Call Manager Express is enabled with DHCP, the dhcp client present in the call manger express initiates a bootp request as a broadcast request to talk with a DHCP server using the broadcast address present in that connected network.

After broadcasting the bootp request, assume that there can be many DHCP server running in the same connected network.
1. In such scenario the DHCP server will identify the incoming request by means of the MAC address (Bootp request is combined with the MAC address).
2. if the MAC address is not present in that particular dhcp's server configuration it will respond with a failure message otherwise the DHCP server frames the required (TFTP address, converts the MAC to assigned IP (Gets the IP address from the IP range maintained by the DHCP server) and other proxy addresses) as a response to the call manager express's bootp request.