MSG-344 Sending fax to extension numbers is not available

const rcsdk = new SDK({
  server: ringCentral.serverURL,
  clientId: ringCentral.clientId,
  clientSecret: ringCentral.clientSecret
})

async function getPlatformLoggedIn() {
  const platform = rcsdk.platform()
  await platform.login({ jwt: ringCentral.jwtToken })
  return platform
}

async function sendFax(input: SendFaxOptions) {
  const formData = new FormData()

  formData.append('attachment', input.attachments[0], { filename: 'correctPDF.pdf', contentType: 'application/pdf' })
  formData.append('to', JSON.stringify(input.to))
  formData.append('faxResolution', 'High')
  console.log('Logging in')
  const platform = await getPlatformLoggedIn()
  console.log('Sending Fax')
  const response = await platform.post(SEND_FAX_PATH, formData)
  console.log('response: ', JSON.stringify(response, null, 2))
}

No matter the number I send, it returns the same error message, and judging by the 400 error, I am correctly authenticated, I tried putting invalid credentials and the error happened before I tried to send the fax out.