The key issue here is passing the value of !!phone into the macro. You might be tempted to use a parameter phonenum, like this:

\define phonelink(phonenum) tel:$phonenum$
<a href=<<phonelink {{!!phone}}>>>Call {{!!title}}</a>

...but unfortunately <<phonelink {{!!phone}}>> doesn't do what we want here – TiddlyWiki does not parse other transclusions inside <<variable transclusions>>. (It's valid syntax, but it will just pass the literal text {{!!phone}} to the macro.)

One option is to put the entire link within the macro, which allows you to use the $transclude widget to pass transcluded values as parameters:

\define phonelink(phonenum) [ext[Call $name$|tel:$phonenum$]]
<$transclude $variable="phonelink" name={{!!title}} phonenum={{!!phone}}/>

Another popular (and more flexible) option is to access the values needed as variables, rather than as parameters:

\define phonelink() tel:$(phonenum)$
<$let phonenum={{!!phone}}>
  <a href=<<phonelink>>>Call {{!!title}}</a>
</$let>

This problem is not limited to macros; it can come up occasionally with procedures as well. Setting a variable is often a useful workaround.