There is one javascript function called "ValidateNo".

In this funciton there are two arguments

1. NumStr : This is the value which you want to validate. It will come from your form       control. It may by TextBox control (HTML or ServerControl).
2. String : This is predefined format which you can use to validate phone/fax number. It may contain +, - and space. You can modify it as per your requirement.

function ValidateNo(NumStr, String)
{
    for(var Idx=0; Idx<NumStr.length; Idx++)
    {
        var Char = NumStr.charAt(Idx);
        var Match = false;

        for(var Idx1=0; Idx1<String.length; Idx1++)
        {
            if(Char == String.charAt (Idx1))
                Match = true;
        }

        if (!Match)
            return false;
    }
    return true;
}


Now create on javascript function called "ValidateDetail".

function ValidateDetail()
{
    if(document.myform.phone.value == "")
    {
       alert("Please specify phone number");
       document.myform.phone.focus();
       return false;
    }

    if(!ValidateNo(document.myform.phone.value,"1234567890+- "))
    {
        alert("Please Enter Only Number");
        document.myform.phone.focus();
        return false;
    }

    return true;
}

Now you can call this javascript "ValidateDetail" function for validating your phone and fax number.