Assuming that the validator is checking the URL against the grammar given in RFC 3966, it's failing on the space characters. Remove those, or replace them with a visual-separator
character (e.g. -
) and you should be okay.
Example:
<a href="tel:0000-00-00-00">0000 00 00 00</a>
Section 3 of the spec has the grammar. In particular:
phonedigit = DIGIT / [ visual-separator ]
visual-separator = "-" / "." / "(" / ")"
See also Section 5.1.1, which says
even though ITU-T E.123 recommends the use of
space characters as visual separators in printed telephone numbers,
"tel" URIs MUST NOT use spaces in visual separators to avoid
excessive escaping.