CRM 2011 Java Script to Validate Mobile Number

function ValidatePhoneNumber_OnChange()
{
    if (Xrm.Page.getAttribute("new_mobilenumber").getValue() != null)
    {
        var PhoneNumber = Xrm.Page.getAttribute("("new_mobilenumber").getValue();
        var bLegalno = isNaN(PhoneNumber);
        if (bLegalno == true)
        {
            alert('Please enter a valid Phone Number without any char');
            Xrm.Page.getAttribute("new_mobilenumber").setValue("");
        }
        else
        {
            var PhoneNumberInitial = PhoneNumber.substring(0, 2);
            if (PhoneNumberInitial != "05")
            {
                alert('Please enter a valid Phone Number start with 05.');
            }
        }
    }
}

Below is the same operation to validate Mobile number and prevent save option if it is not a valid number:

function ValidatePhoneNumber_OnSave(context)
{
    if (Xrm.Page.getAttribute("inf_mob_nonew").getValue() != null)
    {
        var PhoneNumber = Xrm.Page.getAttribute("new_mobilenumber").getValue();
        var bLegalno = isNaN(PhoneNumber);
        if (bLegalno == true)
        {
            alert('Please enter a valid Phone Number without any char');
            Xrm.Page.getAttribute("new_mobilenumber").setValue("null");
            //Stop from saving
          context.getEventArgs().preventDefault();
        }
        else
        {
            var PhoneNumberInitial = PhoneNumber.substring(0, 2);
            if (PhoneNumberInitial != "05")
            {
                alert('Please enter a valid Phone Number.');
                //Stop from saving
                context.getEventArgs().preventDefault();
            }
        }
    }
}

Note:Check the below shown check box .