@simevidas @Edent My thinking was of a similar line, but with the addition of using the Page Visibility API (https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API) as an added heuristic.

If the page is no longer visible after clicking the link then there's a reasonable bet it's gone to a phone app. If it's still visible you've either got something that doesn't handle tel: or has multi-windows. Page Visibility API - Web APIs | MDN