With traditional PBX systems purchasing your handsets is only the beginning. You need to invest in costly PBX hardware to balance and route your available telephone lines to each handset, and any additional features like conferencing require yet more investment in additional hardware that needs to be installed into your main PBX equipment.
The beauty of VoIP is that none of this investment in hardware is required, at all. There's no catch, this is just one of many wonderful benefits that come with technological advancement. Instead all of the heavy lifting being done by traditional on-premise PBX hardware, it is now handled on the "exchange" end of your service, by whichever provider looks after your service.