My boss found out about this telephone amplifier and purchased one for me. It's an HA40 by Clarity. The device plugs directly into the back of my phone, with the phone cord plugged into it.
I was skeptical at first thinking that it would not make a difference. I already had volume control on the phone and usually kept that it in the upper range for the few calls I made or received. But what the Clarity does is increase the high frequency sounds more than the lower frequency ones. Because I have a high frequency hearing loss, this helps the callers sound clearer and easier to understand. The Clarity has two adjustable controls - one for volume and one for tone so I can adjust it for each caller's voice. In spite of my low expectations, I must say it works. I was reminded recently of what a difference it makes when the 9-volt battery inside died. Going back to the standard phone until I bought a replacement battery was a pain. I had gotten used to the better sound quality.
If you are reluctant to use the phone at work like I was and you have a mild to moderate hearing loss, then I would recommend the Clarity HA 40. At less than $50, it's a fairly low-cost accommodation and available at Office Max and other online retailers.