There are two kinds of VOIP (Voice over Internet Protocol) and they can perform quite differently
One is 100% internet where you call from your computer or tablet to an number registered with a VOIP operator- say Skype or Whatsapp . The registering part usually comes from syncing your contacts selecting some to call this way . In this case the call is 10% VOIP and has to be answered byan IP device. Often/usually free
The other is where you have a special phone or a different type of PC app where the call starts out as VOIP and indeed is carried over the internet until it gets to hand off point at the destination end where its ends up back on the local phone network and makes effectively a local call to complete the connection-cheaper than 100% phone service but not free. However you can call any number anywhere .
They both have their advantages but as we saw the other day with Skype they are not 100% reliable and can run into problems with call quality that phone calls do not have -this can be temporary high internet loadings or the interface between the internet and phone network not working properly.
However the trend is towards everything becoming voip especialy from your smart phone which is smart but not the most functional phone in many ways , but think about how many times you actually use it for a phone call.
This could be done now but mobile operators are too busy ripping people off on international calls or special rate numbers to be very proactive However the trend is such that by 2018 I reckon your mobile contract will just be Mbs with voice and text absorbed into that