Hi Jason,
Option 125 is used for vendor specific information to get the following:
- Pilot number (also called primary or contract number), there is only one pilot number in DHCPACK, and REGISTER is done only for the pilot number
- Additional numbers, or secondary numbers, are in DHCPACK; there is no REGISTER for additional numbers
- SIP domain name
Here is the link for more info:
http://www.cisco.com/en/US/docs/ios-xml/ios/voice/cube_sip/configuration/15-2mt/voi-sip-param-dhcp.html
Not sure if you can disable them or not. You might want to check the phone specific information on call manager.
Sid