E.164 is a global FGFnumbering standard for both the PSTN and PLMN. Most VoIP implementations support E.164 to allow calls to be routed to and from VoIP subscribers and the PSTN/PLMN.[27] VoIP implementations can also allow other identification techniques to be used. For example, Skype allows subscribers to choose "Skype names"[28] (usernames) whereas SIP implementations can use URIs[29] similar to email addresses. Often VoIP implementations employ methods of translating non-E.164 identifiers to E.164 numbers and vice versa, such as the Skype-In service provided by Skype[30] and the ENUM service in IMS and SIP.[31]
In India, it is legal to use VoIP, but it is illegal to have VoIP gateways inside India.[61] This effectively means that people who have PCs can use them to make a VoIP call to any number, but if the remote side is a normal phone, the gateway that converts the VoIP call to a POTS call is not permitted by law to be inside India. Foreign based VoIP server services are illegal to use in India.[61]
The caveat there is the proverbial fine print, usually located just below the really attractive dollar figure. This small print generally details exactly how many months that nice number will remain in effect before the bloom comes off the rose and you start getting billed a much higher number that represents the service's actual cost. Many providers don't even print this higher number on their websites, so be sure to ask the sales guy on the phone before you sign up. The nice number that pulled you in can often double or more once the introductory period wears off. Some providers even attach a minimum length of time that you'll need to suffer these higher costs before you can change or modify the service without getting hit with an additional early-termination penalty fee.
VoIP stands for Voice over Internet Protocol. It is also referred to as IP Telephony, Internet Telephony, and Internet Calling. It is an alternative way of making phone calls that can be very cheap or completely free. The ‘phone’ part is not always present anymore, as you can communicate without a telephone set. VoIP has been named the most successful technology of the last decade. 
Yes it is really this simple. Just connect your telephone adapter to your home Internet by connecting a cable between it and your router or modem. Then connect your existing phone to the adapter and you should be good to go. If you decided to use your own adapter, you will likely be required to run a quick configuration script that is supplied by your service provider.