Normally, 90 kbps is sufficient for good quality VoIP (provided, of course, that other factors are favorable too). But this can be rare commodity in areas where bandwidth is still very expensive, or in corporate contexts where limited bandwidth has to be shared among many users.
In case you are a residential user, try to avoid dial-up 56 kbps connections for VoIP. Although it will work, it will very likely give you a very bad VoIP experience. The best bet is a DSL connection. As it goes beyond 90 kbps, you are good. For companies that have to share bandwidth and have to configure their VoIP hardware accordingly, administrators have to be realistic and lower or raise their quality settings according to the real bandwidth available per user. Typical values are 90, 60 and 30 kbps, each resulting in a different voice quality. Which to choose will depend solely on the bandwidth/quality trade-off the company wants to make.
What makes the bandwidth settings adjustable are the codecs, which are algorithms (program segments) that are present in VoIP equipment for compressing voice data. The VoIP codecs that offer better quality require more bandwidth. For instance, G.711, one of the best-quality codecs around, requires 87.2 kbps, while iLBC requires only 27.7; G.726-32 requires 55.2 kbps.
In order to know how much bandwidth you actually have and how suitable it is for your VoIP needs, you can use the many online speed tests available for free. There are tools that are more precise and accurate, for more technical results. An example is this VoIP bandwidth calculator.