In the early days of VoIP, everyone was talking about how the quality was lower than that of a PSTN call with frequent drop offs and delays. At that time, such stories were probably true as the world was still coming to grips with high Internet speeds and VoIP technology in general. However as anyone who's actually used VoIP these days knows, VoIP call quality has not only improved dramatically, it's also superseded that available on normal phone lines. Technology such as HD voice for example has revolutionized the expectations for businesses and VoIP continues to spread.
But what speeds do we need to get excellent VoIP quality at a minimum? The answer greatly depends on what we call "good quality." Regular users expect good quality to be exactly that which is provided by the telephone itself. With the Internet being what it is today, that's the very least you can expect. By VoIP standards, the old telephone voice quality is pretty poor. Normal telephone lines encode voice signals at just 8Khz which sacrifices some of the higher and lower frequencies to achieve good throughput. That is why you can easily confuse certain letters such as "p" and "b" for example.
VoIP however, has the capacity to encode at many levels. Depending on the available bandwidth, a higher or a lower codec can be used. Better quality uses more bandwidth. Some programs such as Skype detect the amount of bandwidth available and choose the codec accordingly. This allows almost anyone to use VoIP effectively.
To get the bandwidth required for a telephone call like quality, we need to figure out how much is needed to carry the equivalent of a 4 khz voice encoding.
It turns out that this is a mere 64 kbps. Except in the most primitive parts of the Internet space, everyone has substantially higher bandwidth than this. So at face value, VoIP calls which are at least as good as PSTN calls are available to just about everyone.
To use better HD voice with VoIP, we encode it at 16 kHz or more. In which case, we need spare bandwidth of around 128 kHz which again in the age of Mbps speeds is very small. Indeed, the limiting factor is usually the fact that not all the participants in a VoIP call are using an IP phone. If they did, everyone would be able to speak in pure HD voice all the time!