Phone app
The phone app uses the same email and password as the website. Calls you make from it show your business number, never your personal one. It runs on Android 12 and iOS 18 or newer; see supported devices for browsers, desk phones, and known issues.
Install the app, sign in, and allow the microphone, notifications, and full-screen calls. Calls for the lines you are assigned to ring that phone, even when the app is closed or the phone is locked. The app is not in the Play Store yet. Install the build you were given.
Install the app, sign in, and allow the microphone. Calls for the lines you are assigned to ring like a normal iPhone call, even when the app is closed or the phone is locked, and calls you make go out from the line's number.
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. More about browser calling.
For the front counter or the office, desk phones ring along with the apps and the browser. Yealink and Grandstream phones set themselves up from one web address, and most other SIP phones work with a few settings typed in.