a suggestion dont know if it will work is to configure the phone as a sip entity on your pbx and then do the following:
The preferred method is to set your IP phones to retrieve the provisioning URL automatically via DHCP Option 66. DHCP Servers can be set to not only deliver IP Addresses, DNS Servers, and Router IP Addresses to network devices, but can also specify a Boot Server Address, which most SIP Phones use to obtain the URL for retrieving configuration files.
Best Regards