The main advantage of IP-telephony is that you can make and receive calls anywhere in the world, the numbers have no geographical reference! While users of traditional telephony can use it only in the office.
One of the main advantages of IP-telephony is that you do not need to buy expensive equipment, so the network can be scaled quickly and easily. Whereas with traditional telephony, you need to buy expansion modules or a new PBX to increase the number of lines, and there is no provision for reducing them at all.
IP-telephony can be set up in 1 day, and the work is done remotely over the Internet and at no extra charge!