@BraswellJay said in What determines the interface that SIP RTP streams over ...:
I finally figured out what was causing my issue.
In FreePBX under Settings->Asterisk SIP Settings there is a parameter to set the External Address used. I had this still set to the public IP of the original interface. Once I changed that parameter to the public IP of the new interface then all started to work over the new interface as expected.
Glad you sorted it out. Good work!