<a href={{{ [[tel:]addsuffix{!!phone}] }}}>Call {{!!title}}</a>

Or, equivalently:

<a href={{{ [{!!phone}addprefix[tel:]] }}}>Call {{!!title}}</a>

This suggests a general fact: most anything you can do with text substitution in a macro, you can also achieve with a filtered transclusion. Which you choose is largely personal preference, although sometimes one might be significantly clearer and easier than the other, in which case you should probably choose that one. And in simple cases like this where backticks will suffice, those are probably clearest of all.