The Dinggly Pager app is available at the App Store or Google Play. You pay an annual fee for the maximum number of concurrent logins required. The app can be downloaded for free to as many devices as you wish, you’re just limited to how many can be active at any one time.
If you are in another app, Dinggly creates push notifications.
To prevent the inevitable issue of people leaving work logged-in, it’s easy to log somebody out remotely.