For voice communication, the bandwidth requirements are more important, since voice is a real time steady stream of data. Broadband is unlimited connection (24 hours a day) at a speed much higher than old dial-up’s (56 kbps dila up was a dream when I was young). Nowadays with ADSL, FFTB and other technologies 512 kbps connection is “so so”, but in most cases it’s more then enough for VoIP communication. For sure all depends on the codec you are going to use and CIR (committed information rate) available form your ISP.
So how to test your link? Put simply, one or more sample files are downloaded from a certain server to your machine through your Internet connection. The file or files are then uploaded again to the server. This activity then provides an environment to measure speeds and other parameters. Prior to starting most speed tests, you are required to select a server, with which Internet activity will be taken for measurement. Some tests don’t give you the possibility of doing so, and assign a server arbitrarily.