Correct - that should work.
Most modern mobile browsers support this, and will automatically launch the telephone app on your phone. On a computer it'll try to launch an installed phone program, usually Skype.
Code
<a href="tel:ACTUAL_PHONE_NUMBER">TEXT_YOU_WANT_USERS_TO_CLICK_ON</a>
So for example; if my phone number would be +581983893 I can just use it as following;
Code
<a href="tel:+581983893">Call Me (+581983893) </a>
If I'm correct using hyphens (-) in your telephone number will be ignored, if you'd like to do that for cleaner/easier to read code.
You can also use an image instead of text by doing this;
Code
<a href="tel:+581983893"> <img src="/path/to/yourimage.jpg"/> </a>
Not quite sure what you mean exactly by "It might mess up my easy template positioning of picture and text?" - As far as I'm aware, it'll just make a link of your phone number. So it shouldn't mess up any positioning at all.