What is VoIP?

VoIP stands for Voice over Internet Protocol. Basically, the analogue signal created by your voice is digitised. Once in a digital format, it cannot degrade with distance and it can be stored in format which doesn't grade with time.
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Once in a digital form, it may be transferred between divergent devices from your VoIP phone, to a hub, internet router, through the various pieces of equipment on the internet to the recipients router and on to their phone.

The HD on a Yealink phone means that the analogue sampling rate is higher than a non HD phone so the audio is clearer at the other end.

With VoIP, it is possible to have many calls on a single internet connection, making multiple lines redundant - you only pay for a single internet connection!

With Audio in a digital form, it may be sent to anywhere in the world with minimal fee so long distance calls cost a lot less.