I have an HX v. 5.1.0.42.
Customer complains that the volume on the phones resets.
I'm aware and they say they are, that going above whatever point resets the volume.
I just raised the CO volume in online monitor for the main complainer, to see if stays there or changes by itself.
Do any of you have this problem, that they say the volumes keep going down?
They do have copper lines.
I'm coming to this problem late and I haven't gone through and checked the volume on the copper lines.

Are you talking about the phone sets themselves or CO lines system wide?
If you have 53XX phones and go to the highest volume level they will revert. Have the customer back down 1 notch and that should hold. On the digital phones, adjust the volume to the desired volume then press both the up and down volume keys at the same time to save the volume.

If it's CO lines maybe you need to run the hybrid balance on loop start trunks.

That feature, of the phones not holding the max volume setting, is actually an ADA compliance regulation.