Welcome to the RingCentral Community

You can find the more details on the below link.

https://developers.ringcentral.com/api-products/fax

Send Fax
The API allows sending a fax message with a multipart request, incorporating two or more parts. The first
part of the request is a general fax message description in json format, including the following parameters: to [phone number, name, location]
Required. Fax message recipients, their names, numbers and locations sendTime
Optional. Schedule time, which is set if the fax is to be sent at a specific time faxResolution
Resolution of the fax message
coverIndex
Optional. Cover page index. coverPageText
Optional. Text entered by the fax sender and printed on fax cover page. The value is limited to 1024
symbols