JSFiddle - React, Tailwind, and code Playground

JavaScript

function IsAustralianTelephoneNumberValid(a_telephone)
{
    a_telephone = a_telephone.replace(/\s/g, ''); // remove all spaces

    // if is empty OR first char is NOT 0 
    if((a_telephone=='')||(a_telephone.charAt(0)!='0'))
    {
        alert("Not a valid phone number");
        return false;
    }

    // lets save the length of that string before we remove digits
    length_with_digits = a_telephone.length;

    // now string has its digits removed
    a_telephone = a_telephone.replace(/0|1|2|3|4|5|6|7|8|9/g,'');

    // if is nothing, then there was no other characters in string
    // except digits and spaces AND ALSO if the difference of length before the digits
    // removal and now is 10 then we can be sure we had 10 digits and nothing else,
    // so its valid. Any other case is not valid.
    if((a_telephone=='')&&(length_with_digits-a_telephone.length==10))
    {
        alert('ok');
        return true;
    }
    else
    {
        alert("Not a valid phone number");
        return false;
    }
}

// 3 invalids
IsAustralianTelephoneNumberValid('11234567891');
IsAustralianTelephoneNumberValid('012345678999999');
IsAustralianTelephoneNumberValid('11234 dang56789');

// and 3 valids
IsAustralianTelephoneNumberValid('0123456789');
IsAustralianTelephoneNumberValid('01234 56789');
IsAustralianTelephoneNumberValid(' 0123 4 56789');