I had the "doesn't work in the rain" syndrome when I was on dial up with Onetel. The dial-up charges mounted and mounted because I was wasting huge amounts of run time in slow or failed downloads. Onetel passed me from one Indian to another. I did get quite a bit of advice on how to set up my computer and although that was educational it did not cure my problem.
The problem was partially cured when the cable from the box at the end of my road to the telephone exchange was replaced. It was better still when the connection from the box at the end of my road to the telphone pole outside my house was replaced. When that was not perfect and I had done all the testing, unscrewing the primary box in my house and plugging several different devces directly into the box, still finding problems, I managed to persuade BT to send one of their best linesmen and his supervisor simultaneously to test things. They replaced the line from phone pole to the box on my house wall and then replaced the line from there to the box inside my house. Only when all had been changed was the problem resolved but it improved at every one of the four steps along the line.
1. Dealing with Onetel was hopeless, both in dealing with the complaint and in billing.
2. Most of the equipment on my route to the exchange is ancient and fails regularly. Probably the same on yours.
3. Persistence in dealing with the problem does pay.
4. For a beautiful customer-provider relationship with English telephone contact, try PLUSNET. They explain technicalities, deal with niggles (very few to deal with) and give immediate refunds if the charging is wrong - this has only been an issue on the telphone part of the bill when the five hours of overseas calls are exceeded and then due to a technicality due to close to end of month calls appearing in the wrong month.
5. I really wish I had had an ombudsman to appeal against the over £100 permonth bills I paid to Onetel. My highest bill since changing has been under £30 for phone at all hours, overseas calls at all hours, and broadband. Typical bill is £24 to £25.