For pure VoIP you need an ethernet card. You can get software VoIP phones that will work with sound cards, but they generally have flaws compared with dedicated SIP phones.
If you have ordinary phones, you no longer have a pure VoIP system, but you can use them, either with, relatively expensive, telephony cards in the PC, or with outboard VoIP to analogue convertors, known as ATAs (analogue telephone adapters).