This can be very simple (and cheap) if you want a basic system where you can make calls and receive calls and don’t want any PBX type functions (transferring calls, holding calls etc.).
You can either install softphones on the PC’s or a small gateway in the office. Quintum and BOSCom offer good solutions as they can be used for outgoing calls via IP and incoming calls via the normal analogue lines so you can keep your existing numbers.
Of course it is possible to go completely IP having your incoming lines/calls delivered over IP as well. We offer a service in the UK with non-geographic numbers to our customers – the calls come to us and we forward on as IP. There are lots of companies in different countries offering this service now so you should be able to find an ITSP near you.
If you let me know how many incoming and outgoing lines you require I’d be able to give you a better idea of pricing and suppliers etc.
Hope this helps