OpalDownload Opal
You'll need a beta account to sign in — request access if you don't have one.
Windows: about the warnings
The beta build is unsigned, so expect two prompts — both safe to dismiss: your browser may flag the download (⋯ menu → Keep → Keep anyway), and SmartScreen shows “Windows protected your PC” on first launch (More info → Run anyway). Installs per-user, no admin rights needed.
macOS: first launch
Drag Opal to Applications. If Gatekeeper hesitates on first open, right-click the app → Open once.
After installing: sign in with your invite credentials, set your phone number in Settings, and you're ready to dial. The app configures itself and updates automatically.