Voice over Internet Protocol

Voice over Internet protocol (or VoIP) is a term used for a number of protocols that are used to carry "voice data" over packet switched networks. Before, telephony used a dedicated line that was only for calls. This was known as PSTN (or "public switched telephony network"). Using VoIP will reduce costs, as the "dedicated line" for telephony (and fax) is no longer needed.
There are a number of technical problems that must be solved first, though:
The vast majority of VoIP systems rely on two main protocols:
SIP :- SIP, or also known as session initiation protocol, has become the standard for multimedia sessions on the internet. The Engineering Task Force thinks of this protocol as the standard for audio, video and even instant messaging solutions. This task force is simply a large, international community, composed of anyone involved in the networking evolution online. SIP is modular, meaning that it can be changed around in whatever way needed. Depending on the type of data your business is wishing to transmit, your SIP deployment will need to be designed specifically for this. With that being said, VoIP and IM communications will need to work around these aspects, and be designed for one total approach. 
H.323 :- This protocol has become the international standard for all communication over packet-switched networks. This includes LAN, WAN and even the general internet networks we all connect to. H.323 is considered an umbrella that includes all types of standards, which is what the internet is based on. H.323 focuses on voice, video, and data communications, designed specifically to operate over IP networks. At this point, this protocol has become the world market leader for voice and voice communications over IP networks. H.323 was developed by the international telecommunications union, which is the organization responsible for building the public telephone network.