Need a 'Click to Call' button that doesn't force https://

This should be such a simple task, but there are Obsticles. 
Trying to add a "Click to Call" button in SharePoint.

You would think that adding tel:0123456789 to the link field in a button Web Part would do the trick, but it doesn't. Our SPO site is forcing https:// at the beginning of the code and this takes us to a blocked page instead of making the phone call.
Have also tried various types of html in the Embed Code Web Part, such as: <a href="tel:0123456789">Click to Call</a>with the same response. I've tried putting it in an iFrame too, still no joy.
Does anyone know the solution please?