Is RingCentral Fax Caller Id only used for the fax header or is it also used to send the fax?

If I update caller id from Online Account Portal to my custom number, will RingCentral use that number (my number) to send faxes or it will only display in fax header and cover page?

Updating the fax caller id setting will update the number used to send the fax. It affects all of the following:

• fax number used to send fax
• fax number shown in fax header
• fax number shown on fax cover page

Your fax caller id number must be available system as shown in the "Available Caller ID Numbers" section on this answer:

Fax from parameter in RingCentral API is missing for sending fax