As a matter-of-fact I did. https://www.amazon.com/gp/product/B0032HM6JG/ref=ppx\_yo\_dt\_b\_asin\_title\_o08\_s00?ie=UTF8&psc=1
Depending on what kind of landline phone you're using, you may be able to plug a headset into the 2.5mm jack on the phone itself. For instance, this headset would work if you can just plug into the audio jack: https://www.amazon.com/Panasonic-KX-TCA430-Comfort-Fit-Noise-Cancelling-Microphone/dp/B0032HM6JG/
If the landline phone doesn't have an audio jack, you might want to ask if you could also get a new landline phone that DOES have one. There are also apparently handset lifters (??) that might be an option: https://headsetadvisor.com/blogs/blog/how-to-connect-your-wireless-headset-to-a-landline-phone
And finally, [obligatory comment about trying to get them to move to VIOP even though I'm sure it's manual due to cost/technological prowess considerations that aren't likely to change].
I hope this helps you as a starting point to find something that might work!