Welcome to the RingCentral Community

I don't fully understand your question but this is the fundamental of sending faxes using RingCentral API.

- When sending a fax, the API does not take the from phone number. The API will take the preset fax number from the user Fax settings

- A user can have his own phone number(s) (a number can be Voice+Fax or Fax Only), and if a direct number is selected, faxes will be sent from that direct number number.

- You can use this API to change the default fax number.

- A user can receive any fax sent to his fax numbers.

Check this dev guide for more info.