function PhoneNumberRequired_ClientValidate(source, clientside_arguments)
{         
    if (membership_telephone1 != null && membership_telephone2 != null && membership_mobile != null)
    {
        var telephone1 = document.getElementById(membership_telephone1);
        var telephone2 = document.getElementById(membership_telephone2);
        var mobile = document.getElementById(membership_mobile);
        
        if (telephone1 != null && telephone2 != null && mobile != null)
        {
            if (telephone1.value.length > 0 || telephone2.value.length > 0 || mobile.value.length > 0)
            {
                clientside_arguments.IsValid = true;
            }
            else
            {
                clientside_arguments.IsValid = false;
            }
        }
        else
        {
            clientside_arguments.IsValid = false;
        }
    }
    else
    {
        clientside_arguments.IsValid = true;
    }
}