JiveTurkey90

Media ports need to be negotiated over signaling to work. If you can find a way to lock down signaling - should be fine.

I know a company that can help if you allow takeover of service vendor of record

I've successfully cutover 3 ipflex services with FreePBX this year. It's a peered connection but they only allow communication with a single IP source public or private is your choice how you want to interface.

$13/user with one I'm familiar with but this comment will get deleted if I say it

try manual provisioning the phone

https://support.ringcentral.com/article-v2/assisted-provisioning-yealink-deskphones.html?brand=RC_US&product=RingCentral_MVP&language=en_US