Get this iPhone's UDID

  1. Open this page in Safari and tap the button. Allow the download.
  2. Settings > Profile Downloaded (top of Settings, or General > VPN & Device Management) > Install.
  3. Safari opens again and shows the UDID. Copy it to the agent.
Show my UDID

The profile only reports the UDID, model and iOS version to this page, which displays them. Nothing is kept installed or stored.