VoIP telephony is of course an existing broadband Internet connection (DSL, cable) installed on your site. For high-quality telephony but here is not the speed of the connection in the first place of importance, but its quality. The bandwidth provided by the companies, so the speed of your connection is always theoretically possible maximum value. Not always the values are reached permanent and often enough it comes at the expense of connection quality. Especially at peak times when many users surf at the same time there is traffic congestion, which can cause a negative effect in the IP telephony.
Note that the IP telephony is executed in real time, ie Data packets due to a congested internet connection does not time can not be requested again to reach the other side of this, as it happens when browsing or while downloading. The data packets are therefore irrevocably away which makes itself felt as a slight "crackling" in the line, to larger voice dropouts.
For a single call is about 80kbit / s of bandwidth needed. With a DSL connection 2000s about 20 simultaneous calls, however, it would not automatically possible because the speed rating of your DSL connection the download direction only thinks. The upload direction always here has a much lower bandwidth, in a 2000 connection, it would include only 768kbit / s The actual number of concurrent calls so always needs to be calculated by the upload speed.