Something you are doing with your CSS was getting in the way. The link you provided was not at all clickable in Chrome, Firefox, or Safari.
I dropped this piece of your code in an email:
<table width="100%" bgcolor="" border="0" bordercolor="" cellpadding="0" cellspacing="0">
<td style="font-family:Arial; font-size:13px">
<a href="tel:800976004" target="blank" style="border-style: none;">
<img src="http://image.crm.kaplaninternational.com/lib/fe9a13707567017e76/m/4/n.verde.jpg" style="display: block; margin: 0px; border-style: none;" alt="Kaplan Home Page" height="60" width="600">
And it worked immediately. I am able to click this on my mobile phone, and in all browsers.
After further investigation - you seem to have
pointer-events:none in your CSS. I disabled that, and it seemed to work immediately as well.
The documentation on
none The element is never the target of mouse events; however, mouse
events may target its descendant elements if those descendants have
pointer-events set to some other value. In these circumstances, mouse
events will trigger event listeners on this parent element as
appropriate on their way to/from the descendant during the event
If that isn't required - I'd remove that and see if it works for you.