Finally figured it out. The Check Point firewall was modifying the SIP in what they call the "fw early SIP NAT" process. This can be disabled following this SK Article: How to disable 'fw early SIP nat' chain / SIP inspection. As soon as we pushed policy to the firewall, the phone registered. Thanks for the help.
Great news! Glad you got it figured out! Thanks for posting the resolution.
Have a great week!