_googWcmGet callback not working via GTM with dynamic phone number

Question

I'm trying to get Adwords Call Conversion tracking working on my site, using GTM. Everything is set up on the Adwords side (call extensions + call from website conversion) but the replace function still isn't working.

<script type="text/javascript">
// the code from adwords, loads the _googWcmGet function
(function(a,e,c,f,g,b,d){var h={ak:"XXXXXXX",cl:"XXXXXXXX"};a[c]=a[c]||function(){(a[c].q=a[c].q||[]).push(arguments)};a[f]||(a[f]=h.ak);b=e.createElement(g);b.async=1;b.src="//www.gstatic.com/wcm/loader.js";d=e.getElementsByTagName(g)[0];d.parentNode.insertBefore(b,d);a._googWcmGet=function(b,d,e){a[c](2,b,h,d,null,new Date,e)}})(window,document,"_googWcmImpl","_googWcmAk","script");

// my callback to replace the number
var replaceNumber = function (formatted_number, unformatted_number) {
  console.log("replaceNumber() called");
}

var getNumber = function () {
  var links = document.getElementsByTagName("a");
  var oldNumber = null;

  for(var i=0; i<links.length; i++) {
    if( links[i].href.indexOf("tel") > -1 ) {
       var el = links[i];
       var oldNumber = el.innerHTML.split("<")[0];
    }
  }
  // console.log(oldNumber);  -> 123-456-7890
  return oldNumber;

}

window.onload = function() {
  _googWcmGet( replaceNumber, getNumber() );
};

</script>

getNumber() is returning the correct phone number to replace (format: 123-456-7890), but replaceNumber is never being called. This looks pretty much exactly like the example, except that the phone number is retrieved by a function instead of provided statically.

Why isn't replaceNumber being called?