Guest Checkout problem

[mcbole62]

I've recently been getting some complaints from customers who are trying to purchase something using Guest Checkout.   I finally figured out that they are only seeing one Telephone field at the bottom, where it says ** You must register at least one phone number.   The Mobile Phone shows but not the other one.  You enter a phone number and you get the "phone is required" error message.  I've tried looking for the file that contains the instructions for this page so I might fix it, but I have no idea where to look.  Can anyone else tell me where this file is that controls the guest checkout address and phone information.  

Using version 1.6.1.17  and there are no updates that I can see available...

 

Thank you...

Matt