Equipment and bandwidth quality will affect sound quality, though there are some computer and network settings I know of that can help boost things (it won't solve the root cause of the issue, but it can help compensate for it a bit). Specific problems can have different causes.
For example, static on a VoIP connection usually indicates the call is being translated to analog somewhere, since digital signals don't really have a lot of opportunity to introduce that kind of interference. This could mean someone is on a DSL connection with a poor quality line or a bad filter. In this case, you can try moving your DSL modem to a different port to see if you get a better connection, or you can try replacing your filters (or just unplugging those connections one by one until you find the bad one).
This is just one example for just this problem. There are additional causes and additional possible fixes for each, and the same is true for most other types of problems. Knowing the type of problems that are being encountered most frequently can help narrow down which would be the most useful to talk about. I might end up making my article a multi-part series to cover more possibilities (it'll be a challenge finding the time to get that done!).
There are also tests you can do for your connection to help diagnose problems, provided you know where they are and what to look for. The tests alone won't fix the issue, but they can help you determine if there's a problem in your router or with your ISP, and knowing where the problem is is usually 50% or more of the process of solving it. I can probably write a whole article just on identifying problems without even addressing how to fix them.