About This HTML Issue
This tel: link has a double quote (") inside its href value, and " is not allowed in a URL.
The usual cause is quotes around the phone number instead of around the attribute value. In <a href=tel:"555-0100">, the value itself is unquoted, so the HTML parser keeps both quote marks as part of it and the URL becomes tel:"555-0100". For the same tag, the validator also reports “"” in an unquoted attribute value. Writing href='tel:"555-0100"', or using " entities, gives the same result, because " turns into a real " before the URL is checked.
For the number itself, RFC 3966 allows digits, a leading +, and the visual separators -, ., ( and ). Quote the whole attribute value and keep the number bare inside it. The link text can show the number in whatever format reads best. A value that holds two numbers has to become two links. Tel: with a capital T is fine, since scheme names are case-insensitive, though most pages write tel:.
Invalid example
<a href=tel:"+1-555-0100">Call us</a>
Valid example
<a href="tel:+1-555-0100">+1 555-0100</a>
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