I have a HH5a and it has only done something like that when something very strongly impacts the signal quality on the line itself. Went through everything inside the house and was found afterwards to be something between house and cabinet (so outside of our control and in hands of BT engineers who bothered to spend time finding the fault). But every time we got someone to fix one area, a few months later another section flares up on the voice line. But the signal issues on my graphs were only for minutes at a time moments and not hours on end like yours. Which might mean you're getting something constant if the voice line is quiet, so it could be something like crosstalk interferrence from another line that connected recently and now it's acting up non-stop (if it has lasted longer than what is seen in those graphs).
But if it is crosstalk interferrence, there's nothing you can do about it if I'm not mistaken. If you're getting any disconnections or concerned it might lead to something worse later on, you might want to consider grabbing a cheap modem/router that holds the signal well, and see if that can keep the line better than the HH5a. I myself grabbed a cheap TPLink W9970 (v2) and just plugged it in instead of the HH5a and never had any major issues since, even when there were line noise issues (ours has flared up again recently, and it very noticable now, but the W9970 kept the line up and steady no issues). The HH5a is getting a bit long in the tooth now, and updates were very sparse a few years ago and it dropped line connections the moment anything fluctuated, so I wouldn't be surprised if there's no update to resolve or reduce the issues you've encountered, and possible hardware faults developed over time might have crept in too. So a change of device might be good for you in any event.