That is true but in theory you can do IP to IP calling with the correct software on your PC. EG Softphone should work aswell.
But if you looking to have true VoIP you need another bit of kit like an SPA3102 also known as ATA's .
These convert your POTS calls into VoIP calls etc. it is money well spend to get one of those.
Have a look at the FAQ on the 3cX website there is a lot of information you can use to find out what is best for you. There is also a picture somewhere that oultines very clear what you need to make it work.