While there are significant differences between the three protocols, they all depend on the same set of conditions in order to work properly. Typically, the only analog connection between your modem and your ISP is the one between your home and the phone's switching office. Phone Lines V.90 depends on high quality phone lines between your modem and the local phone company's digital switch. Any noise on this portion of the lines will result in deteriorated performance. This 'noise' is not necessarily audible sound.
All phone lines carry some degree of 'noise'.
Unfortunately, local telephone companies offer no guarantee of line quality, where data communications are involved. The 'official' policy, depending on who you talk to at the phone company, is that they only support something between 2.4K and 9.6K connections. If you suspect that line noise might be giving you problems with your connection, Coinet recommends that you do try contacting your local telephone company. If you hear any audible noise on the phone line, they should take measures to eliminate that. The phone company seems to also respond better if they get reports of problems sending faxes.