Add the "tel:" prefix to the linkInternalFilters variable in your Adobe Analytics config.
Example:
s.linkInternalFilters = "tel:,javascript:,mailto:,.yoursite.com";
If you have Adobe Analytics implemented as a Tool in DTM, then you can add it within the AA tool config, under
Link Tracking > Outbound Links > Never Track

The DTM config version above will also populate linkInternalFilters.
And in general, linkInternalFilters is a substring ("contains") match against the href=".." value. So it's not the same as using regex or other code to ensure the href value starts with "tel:" but in practice, this is usually good enough, because it will match your tel: links and usually you won't have other URLs where "tel:" is part of the href value but not at the beginning.
But if you really want to ensure it is at the beginning, then within your doPlugins function, you can suppress the s.tl call with the following:
if ( s.linkObject && s.linkURL && s.linkURL.match(/^tel:/i) ) {
s.linkType = 0;
}
Or, if you don't want to suppress the s.tl call, but want to keep the telephone number from being recorded, you can set s.linkURL to a different value:
if ( s.linkObject && s.linkURL && s.linkURL.match(/^tel:/i) ) {
s.linkURL = 'masked value';
}