Marketo Encoded URL

You would need to add ALT TXT to the image. Click on the image and then image editor icon.

For the telephone, you need to add mktNoTrack to the Class section of the URL editor.

We have alt text. The hyperlinked URL appears. Would I have to add the mktNoTrack to everything? We want the URL to be tracked though, just not appear when you hover over the image.

The closest to this kind of obfuscation (or "friendliness," if you will) is the TITLE attribute, not ALT.  The ALT is designed to be displayed in place of the image when the image is not available or when the device does not support images, not as a tooltip above a successfully loaded image.

However, bear in mind that it is up to the mail app (in which I include both browser-based mail and HTML-aware standalone apps like Outlook) to decide to show you the TITLE as a tooltip.  Many versions of Chrome do not render the TITLE as a tooltip and Google may well discontinue its use because it isn't accessible; mobile browsers also are under no obligation to continue this non-standard behavior from the olden days; and plenty of standalone apps don't interpret TITLE at all. And, regardless, modern browsers will show the actual target URL in their Status Bar as a security measure.  There is no way to fully conceal the HREF of a standard HTML A tag (of course you can do whatever you want via JavaScript but that is rather far afield from usable HTML email).

I guess I also wonder what you expect to gain from seeing a tooltip during testing. It doesn't actually guarantee that the link is "correct," as the URL could still be wildly mismatched from the TITLE and you wouldn't know it. Can you explain in more detail the use case for this?