There were Merlin handsets made with a volume wheel on the underside of the handsets, but I don't know if there was a confidencer in the microphone part. You might try the sponsor of this website by clicking on the Equipment link at the top of the page, to start with.

It might be cheaper in the long run to put in the F.B.I. and be done with it. Depends on how many phones you have in the shop. Also, the F.B.I. will work with any system if they ever decide to upgrade from the Merlin. JMHO.

Rockford VoIP