===EDIT 2===
It is now official, I have wasted a chunk of my life trying to solve for something that already existed (despite SFMC support saying it did not). It has tested with no issue across every major client/mobile device without any issue. Worst case was that the button would force a browser window open and then pop up a confirmation notification prior to moving it to the phone number.
===EDIT===
So, I just tested it with RedirectTo() 'RedirectTo('tel:15555555555')' around it and it seems to work....SFMC support told me it was not possible, but it seems like it actually is. You can likely ignore most of what i say below. I am still verifying across each client and phone, but it looks good as works on latest Android and iPhone across popular clients.
I have been trying to solve for this for a while as well and really there is not a good option. If you are able to track via your telephony system or something, that would be best as removing this need from the restrictive email environment is the best solution by far.
First: you need the 'tel:' protocol in front of your phone number. e.g. 'tel:14254981234' and secondly, redirects for this seem to be a non-native capability for emails as redirects to tel protocol are unstable. On websites, you can use click events via JS to solve, but email does not support JS.
You could try to use the :active inside CSS, but this is not widely supported and would be inaccurate.
The best (in terms of not being creepy or delving into a grey area) solution I have found is to have the click link to a webpage with a second click option on it. This is pretty crappy UX though and will likely decrease conversions.
The best (very shady) solution I have heard is to build a LP that has every option available to redirect on it (meta tag, body onload, and build a JS click() function to click a link to the phone # on load, etc.) and have Server side scripts to collect the tracking info. It works, mostly...but is not at all something I would recommend as it will likely get you in trouble somewhere and has potential to break or give a bad customer experience.