I am looking for a device that I can put between a phone and a wall. 
It needs to deliver a prerecorded message to whoever is calling in
then forward the call to the phone.

We are looking to use this device at each of our locations to inform
callers about the website they can use before forwarding it to a live
body.

Hi, there was very recently posted a question that asks for
confirmation that many people hang up when they get a recorded
message.  The message would have to be very well designed to keep me
from doing so.  You might check it yourself.

It is actually for gated communities to put their guests on the allow
list.  So if they want their guest to get in they will either need to
follow the recording and do it online or wait and get the guard on the
phone.  It will probably be a very short message anyway.

If you have a separate line for the phone and for the device (eg
extensions off a small PBX) an automated attendant like the Viking
ACA-1A will probably do what you want. Alternatively a small Call
Sequencer like the TMS-2 might be suitable for your application. "In
the day mode, the TMS-2 answers calls while you are busy or on another
line. After your personally recorded, professional greeting has been
announced, the TMS-2 holds the caller until you are available to take
the call. Music on hold can be provided from a radio or another audio
source." I suggest calling them to discuss your application.
www.vikingelectronics.com

Owain