I want a screening device to put on the phone that will answer "please
enter code number to put your call through" and then if they do, the
phone will ring.  if we are not at home, the answering machine will
answer.  Can you do it?

Most phone companies offer a service called "Privacy Manager" where
all callers have to say their name.  Then your phone rings, and you
hear the name of the person calling and decide whether or not to
answer it.  This does not keep the phone from ringing but should deter
unwanted calls, and lets you know who is calling before you commit to
talking to them.

Actually, I just found exactly what you're looking for.

"Two manufacturers of such products are www.command-comm.com at (800)
288-3491, and www.avinta.com at (800) 227-1782 (no endorsements
implied)."


This is from http://www.privacyrights.org/fs/fs3-hrs2.htm

Here is a link to a web site that offers a phone screening device
machine that provides the functionality you require:

http://www.home-and-office-phone-depot.com/pages/products/trivox-vn100.htm

The product is called the TriVOX VN100. According to the product FAQ
provided, you will need to hook up a dedicated answering machine to
answer the calls you are truly away.

The FAQ and a comprehensive product description can be found here:
http://www.home-and-office-phone-depot.com/pages/products/trivox-vn100-faq.htm#faq

The web site offering the product for sale is called Home & Office
Home Depot. The email address is
support@home-and-office-phone-depot.com and the toll free phone number
is 888-633-5777.

A link to their 30-day money-back satisfaction guarantee is provided
here:
http://www.home-and-office-phone-depot.com/pages/guarantee.htm

Thank you for your suggestions.  I will investigate them both.