I use dialup. I find that my connection speed seems to depend first on the quality of the phone line, second on the modem and computer that I'm using. As far as I can tell, my ISP's equipment has never been the limiting factor.

Some ISPs are more helpful than others about connection speed issues. I've found quite a few articles on the subject online, some apparently written by an ISP for their own customers but accessible to everyone. Here's one; here's another. I'm sorry to say they do get pretty technical in places. I'm a nerd so I don't mind, but probably not everyone will want to dig into the subject quite that deeply.

In theory the way out of dialup probems is to go to DSL, cable, or satellite but those seem to come with their own problems, not to mention costs. So far, whenever my phone line has acted up it's always gotten better after a few hours or (at most) days.