There are three major advantages of using VoIP(Voice over IP):

1. Cost:
   Generally phone service via VoIP is free or costs are less than equivalent service from traditional sources. The reason behind this is the use of single network to carry voice and data. If you have established network then you can use VoIP at no additional cost. VoIP to VoIP phone calls on any provider are typically free, whereas VoIP to PSTN calls generally costs the VoIP user.
   
2. Functionality:
   VoIP offers various facilities which may be more difficult to achieve using traditional phone networks.
   You can receive incoming calls to your VoIP phone regardless of where you are connected to the network. Therefore you can take your VoIP phone wherever you wish and you can receive incoming calls if you are connected to internet.
   Using VoIP phones, call center agents can work from anywhere in the world.
   Many VoIP packages include PSTN features that most telcos normally charge extra for, or may be unavailable from your local telco, such as 3-way calling, call forwarding, automatic redial, and caller ID.
   
3. Mobility:
   VoIP allows users to travel anywhere in the world and still make and receive phone calls.
   Subscribers of phone-line replacement services can make and receive local phone calls regardless of their location. For example, if a user has a New York City phone number and is traveling in Europe and someone calls the phone number, it will ring in Europe. Conversely, if a call is made from Europe to New York City, it will be treated as a local call.
   Users of Instant Messenger based VoIP services such as those offered by google talk, yahoo and msn messenger can also travel anywhere in the world and make and receive phone calls.
   VoIP phones can integrate easily with other services available over the Internet, including video conversation, message or data file exchange in parallel with the conversation, audio conferencing, managing address books and passing information about whether others (e.g. friends or colleagues) are available online to interested parties.