I know this is a bit of a zombie but I'll add my experience with BT.... Its a long un.
Back in November last year we suddenly became unable to send an email. Our download speeds were fine but upload was virtually none existent. This meant all our devices stopped working properly (Alexa, smart lighting etc) but worse still, WFH DH couldn't upload files.
We called BT who brushed us of with 'well your download is fine and thats all we guarantee'. Took some digging but finally found something in their small print guaranteeing a 'service'. Grudgingly, they sent out an engineer.
After the fifth engineer with no improvement BT actually turned around to us and said "Its your fault. You have 29 connected devices to the router, you're slowing it down, we're doing no more but you can leave us if you want to". We pointed out that the upload speed remained the same with nothing connected to the router at all, and we left.
We moved to Talk Talk on the basis they guaranteed 250 connections to their router. Instantly obvious there was no change. Took THREE WEEKS to actually speak to someone. Had an email that wasn't even in any form of English. We left.
It's March by now and we still cant send an email. We go to a company called Andrews & Arnold. They guarantee to fix all problem lines. Its more expensive and we lose our phone line but by now we are desperate.
We go through several more Openreach engineers who want to tell us it's our fault, but the difference is A&A keep badgering them, keep monitoring the line and can see it's not us. They even send out a REIN Engineer to see if there is interference from somewhere, there isn't.
Finally, at the beginning of this month 2 more engineers turned up, went to the cabinet at the end of the street, swapped over the connections and our upload speed shot back up. It wasn't us, it never was and had BT done this back in November they would have solved the issue, rather they couldn't be arsed, saw us as complaining customers and shut us down.
The moral is, BT are shocking, Talk Talk almost as bad and if you have a problem line go to Andrews & Arnold. Worth every penny.