1. Customer pays for RingCentral Professional Services to develop a custom integration for them (please contact your Account Executive for more info).
2. Customer develops a private integration for their own usage if the target provider offers an open API as well (since RingCentral already does, details of which can be obtained on the RingCentral developer portal).3. The target provider develops an integration with RingCentral as a Connect Platform Partner that is available in our App Gallery.
I hope that helps!