I'd already tried adding 'tel:' to 'Special Link Prefix' in CQ Link Checker Service configuration. I've also tried adding x-cq-linkchecker="skip" to the <a> tags. I'm using AEM6. I'm using a sightly template - does this x-cq-linkchecker="skip" work in Sightly?
I've followed your steps and added a com.day.cq.rewriter.linkchecker.impl.LinkCheckerImpl under /apps/<myapp>/config, and no luck!
I've also tried turning off linkchecker entirely by disabling it in Day CQ Link Checker Transformer. And this has no effect!