Hi Dermot,

We have a number of similar users here, and have used the Walker amplifier (W-10) quite successfully. With this amplifier connected you don't need a separate handset just use the standard Cisco handset that was shipped with the phone. The handset is Hearing Aid Compatible (HAC) and meets FCC loudness requirements for Americans with Disabilities Act (ADA). Section 508 loudness requirements can be achieved using industry standard inline handset amplifiers such as Walker Equipment W-10 or CE-100 amplifiers.

From this 7940 doc;

http://www.cisco.com/en/US/products/hw/phones/ps379/products_data_sheet09186a008008884a.html

This is also a very cost effective solution. Have a look (just an example :)

http://www.cdw.com/shop/products/default.aspx?EDC=485315

Hope this helps!

Rob