Reading your posts, this sounds very similar to the problems I was having back in 2012 when the broadband would cause crackling on the line, but it never seemed to happen/be nearly as bad with the broadband
disconnected. One of the things that we found constant was the ability to hear the modem 'talking' to the DSLAM when syncing up (Robotic sound on the line). If this applies to you, it will make things much easier since you can show this to the engineer when he's on-site.
We only managed to get the problem diagnosed by communicating with the UK forums team at BT who sent out multiple engineers.
Unfortunately in this modern day and age, you get
some (Select few) engineers at Openreach that are near useless. These few 'engineers' won't be proactive, will not deviate from their way of doing things and will do as little work as possible, even if it means feeding rubbish to the end user. Due to the way the industry is regulated (Lack of in some respects), a lot of the time you rely on chance as to whether you'll get a good engineer and/or an engineer with the right equipment.
It took us multiple engineers to get our line sorted in the end and the problem was only diagnosed when we got an engineer with the correct equipment (PSTN Engineer) for tracing the problem which turned out to be a [censored] 'tee' joint connection that was a 'direct burial'.
In our case, I found myself constantly having to relay information told to me by the engineers to the support team at BT because Openreach were not capable of communicating issues between engineers internally and every time we took one step forwards, we had to take a step back.
At the end of the day it all boils down to the fact your contract is with PlusNet. You really need to get into contact with someone there whether it be on the PlusNet forums (Not sure if they have a dedicated support team like BT) or on the customer service team there.
One thing I would say;
try and get into contact with a support team that uses email! This makes it a lot easier to communicate issues.
As I'm sure you're aware since you've already stated this, you need to push this as being purely a voice fault. I've found that PSTN engineers are typically better equipped than Broadband engineers at Openreach, so they will be more likely to find the issue and you're more likely to get one for a voice fault. When they do, talk to the engineer to find out what's going on. To be safe I would tell PlusNet immediately what the engineer has told you in-case the message isn't passed along within Openreach (Quite frequent from my experience). That way they can chase OR for a resolution.
Hopefully someone on here has information on a written support team within PlusNet?
Sorry for rambling on but I wanted to get everything down in-case any of this is at all useful for you.
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A.K.A: Chrisszzyy
Telewest (2004-2006): 256Kbps -> 512Kbps
University of Portsmouth's Horrible Network (2013 - 2014) - Supposedly 100/100Mbps
BT (2006 - Present): 8128/448 -> 22494/1211 -> 79987/20000Kbps (BT Infinity 2 on Huawei Cab)
Virgin Media's ridiculously rubbish upload connection (2014 - Present): 152/12Mbps