any advice?!…

im searching for a software that i can use my computer as a telephone over a local office network..i just found those who works over internet protocol (VOIP) and its not what i want..i even have this question, is there any software that i can use on every computer in the office as a telephone? for internal and external calls? my plan is connecting the main computer to a telephone line so i can receive the incoming calls on it and from it i would be able to transfer the call to any employee’s computer after installing that software on every computer n the network, and also for internal calls between every computer on the network.
i would be glad to hear from experienced people and appreciate any advice..