Could you offer me a guide on implementing VoIP in my office? I want to know how can I implement the software using (2000 servers, Linux or isa) instead of PBX.
What you are looking for is a softswitch. It will act as a bridge between your PBX or the PSTN and you IP voice services. Conversations that can be carried on the IP side will be routed over IP and the others are simply connected to the PSTN. There are several on the market.
Another option is to discuss this with your current phone company. They are offering VoIP services in increasing numbers. They may have the gateways installed at their central office providing you with additional savings.
I don't know if you currently have a PBX, but it may be possible to get a gateway for your existing equipment as well. Your equipment vendor will let you know.
That said, you will want to make sure that you have the bandwidth available for this implementation. Voice traffic is rather picky about throughput. A good network audit will assist, and provide you with the information necessary to do capacity planning for your implementation.