You can send a fax using a different caller ID. To send a fax using a different extension you would have to log into the extension that you want to send from.
Beyond callerId, if you actually want to send and store as a different extension, you would need an access token for each extension you want to send/receive as.
John Wang, Official Rep
https://stackoverflow.com/questions/50099677/fax-from-and-callerid-parameters-missing-from-ringcentr...
Beyond callerId, if you actually want to send and store as a different extension, you would need an access token for each extension you want to send/receive as.