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

Or alternatively:

<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.