A phone-number QR code contains a tel: link. Scanning it opens the phone dialler with the number already entered, so the person only has to press call. It is a small change that removes real friction on 'call for assistance' signs, equipment service labels, property listings, delivery notes, taxi cards, emergency contacts and the back of a business card.
Use the full international format with a leading plus and no spaces or separators: +15551234567 rather than (555) 123-4567. Most diallers can handle local formats, but the international form is unambiguous when someone scans your sign while roaming, and it keeps the code short and easy to read.
As with any printed code, add a caption - 'Scan to call support' - and print the number in plain text underneath as a fallback for devices that cannot place calls, such as tablets and laptops. Export SVG for large signage so the edges stay sharp.