When the phone first boots up it is technically a dumb device. It has no configuration or settings applied. Somehow the phone must be told where its Call Manager is so it may go request its configuration file. The main way most people achieve this is by putting the location of the Call Manager in what is called an IP option.
When the phones gets an IP from the DHCP server it is also given an extra field with a value ( in your case this would be the IP of the call manager). These extra fields are called DHCP options. They are handed out as an Option # and value.
you configure them under the system services dhcp pool hierarchy
ex)
set system services dhcp pool 192.168.0.0/24 option 43 .......
The phone will only listen for the proper option number so you must get this from the manufacturer specs.
Alternatively you can just allow the Call Manager to be the DHCP server and setup a DHCP forwarded in the Voice VLAN. This is typically the model I choose because it removes a layer that the "Network Team" has to support. All voice stuff lives on the voice server.