Android
Install the app, sign in, and allow the microphone, notifications, and full-screen alerts. Calls for the lines you are assigned to can ring that phone. The app is not in the Play Store yet. Install the build you were given.
Phone app
Android rings. A locked iPhone does not, yet.
The phone app uses the same email and password as the website. There is no server address to type.
iPhone
The iPhone app can sign in and place calls while it is open. Ringing when the phone is locked needs Apple’s paid developer program and a VoIP push credential on your line. That work is not finished. Do not buy Desk if a locked iPhone is the only way you answer.
Browser
You can answer with no phone app. Stay signed in, allow the microphone once, and keep the browser ready. Incoming calls ring on any page of the desk. If the registration drops, the keypad offers Try again. Closing the tab unregisters it, so the line is not stuck showing you as busy.