Last year I was interviewed over Skype, this being the choice of the BBC sound engineer although I told them our landline was very clear. Absolute nightmare -- there was a 1.5 second lag on the line making the flow of question/answer almost impossible.
Skype is an essentially free system which can suffer from congestion, a downside of the free nature, and the volume of users. It is a propriatory form of VoIP that can't interoperate with systems based on standards; this is because it was early.
for quality voice, your broadband download speed is usually not an issue, if your uplink is slow (under 2 Mbps) then it is advised to ask other users to stop using whilst you are on a call.
Some radio stations were using WhatsApp and Facetime Audio to call in, the broadcast output quality of these calls was dramatically clearer than any analogue telephone line.
The reason it sounds better is that old copper phone lines were designed for voice and apparently only carries 8 kHz. A VoIP connection over the internet can choose different codecs starting at 8 kHz upto 44 kHz which would be CD quality.
21 years of broadband connectivity since 1999 trial - Live BQM