Every phone number on the page, checked for a tel: link with the right digits, a call action visible without scrolling, and agreement with the JSON-LD telephone.
Read the story behind this tool: Why Click to Call Audit exists.
Related: NAP Consistency / Scroll Depth + CTA Visibility Map / Mobile Parity
Run it on the page a phone visitor lands on first: the home page, the contact page, or a location page. One fetch per run.