There is a thing called
contention ratio that looks at the number of folks connected to a phone line. The logic is that it can only carry so much data & if you're at the end of the phone loop, everyone else will "nick your space".......it's just the old water debate for the 21st century!
I can hear the old f@rts in the village now, complaining that Frederico always eaves his connection on, meaning that Walter can't get any data........pishaw!
So what can you do *if* it is a contention ratio problem?
This article is quite useful as it suggest a number of good options but I guess that your problem is not the number/nature of the other users but a poor quality service (or maybe even local 'phone exchange) and that will involve lots of calls and you could end up in a lot of buck passing......