Welcome to the RingCentral Community

Since this call was initially asked and answered, we have released an API for managing Call Forwarding and Call Flip rules and phone numbers. We suggest you consult the following documentation:

https://developers.ringcentral.com/guide/call-management/manual/call-forwarding

There is an advanced-level API to manage forwarding numbers (which you may not be able to see in API docs). But you can try (it requires EditExtension app permission):

POST /restapi/v1.0/account/255537016/extension/255537016/forwarding-number HTTP/1.1
Content-Type: application/json
{
 "phoneNumber": "+16504326573",
 "label": "Custom number"
 } 

There are also APIs to modify and delete forwarding numbers (usual PUT and DELETE). You can send a support request to get access to full documentation about forwarding numbers API (and other advanced APIs) for your account.

Thanks for the quick reply.

Can you explain a little more about what that means? Does that require manual input?

Not via the API, but this can be done from within the Online Account Portal (https://service[.devTest]. ringcentral.com).