After some nice discussion in the community meeting, it looks like the “Tel:” prefix is being injected so that the numbers can be shown as a link by the browser renderer. This gets put directly into the clipboard when the “url” is pasted. Here’s the place it seems to happen, and I think it’d be an easy fix, in case anyone fancies proposing a PR to do so: https://github.com/sailfishos/sailfish-components-webview/blob/master/import/popups/ContextMenu.qml#L164
It’d probably be worth fixing for mailtos at the same time. The type of link has already been identified by the code, as can be seen here: https://github.com/sailfishos/sailfish-components-webview/blob/master/import/popups/ContextMenu.qml#L109