to complete the story, i'll add is_numeric function telling if parameter consists solely of decimal digits. i checked my config and found that it contains more of those tests than is_tel_number tests.
regarding is_tel_number test, it currently returns true on "+" argument. i would suggest that at least one digit must follow + sign.
also, while checking the list of existing siputils functions, i found these two be redundant:
is_uri_user_e164(pseudo-variable) is_rpid_user_e164()
i suggest that they are removed.
-- juha