Sending faxes over VoIP networks is sometimes referred to as Fax over IP (FoIP). Transmission of fax documents was problematic in early VoIP implementations, as most voice digitization and compression codecs are optimized for the representation of the human voice and the proper timing of the modem signals cannot be guaranteed in a packet-based, connection-less network. A standards-based solution for reliably delivering fax-over-IP is the T.38 protocol.
One important advanced feature that's ubiquitous in the world of business VoIP services, and quickly growing in the residential market, is the softphone app. Imagine a piece of software that simply uses the network connection, speakers, and microphone of your computing device to turn that device into a phone. If that softphone is attached to your VoIP account, that software will ring whenever your home phone does and when you place calls on it, those calls will register as coming from your home phone number. Just by installing the software you'' be able to immediately place and receive voice calls over your home phone account on your PC, your Apple iPad, or even your smartphone. That last one is a gotcha, however.
In IP telephony, no such direct link between location and communications end point exists. Even a provider having hardware infrastructure, such as a DSL provider, may know only the approximate location of the device, based on the IP address allocated to the network router and the known service address. Some ISPs do not track the automatic assignment of IP addresses to customer equipment.[34]

Phone Power is another home VoIP provider that runs its service using an on-premises device. This is called the Home Adapter and like other services, it sits between your phones and your Internet connection, though no other network is required. It can even function as a router on its own. While it's not the cheapest home VoIP solution we found, it's certainly well-regarded and mature with a wide variety of options and capabilities.
It gets even better! Calling internationally using VoIP is far less expensive than using a traditional landline phone company. Every region of the world, from Afghanistan to Zimbabwe, has its own combination of technological development, geographic obstacles, regulations, and taxes. Cell phone carriers, much like VoIP providers, have their own network, which is why it usually costs more to call international cell phones. If a VoIP provider has a significant presence in a country, it’s likely that landline calls will be unlimited with an international package. For example, if you are using Vonage Word, calling from the U.S. to a landline in Germany is included, but it costs US$.0061 per minute to call a cellular phone. If you are using ViaTalk World, you can call both German cell phones and German Landlines for the same rate – free up to 1200 minutes per month. Don’t forget that with every VoIP provider, if you call within their own network, calls are always free no matter where you are in the world. With the development of SIP, you can easily make and receive free international calls from your home using VoIP.
While understanding the basics of VoIP and SIP is important, setting one of these systems up will require some general network knowledge, too. For the best quality, you will need to meet a minimum upstream and downstream data throughput requirement. In addition, you'll also need to meet a minimum latency number (that is, the time between when a signal leaves a remote computer and when your system receives it), typically measured in milliseconds. It is possible to test your network connection to see if it will support a VoIP service. RingCentral offers this service from their website, other vendors like to have their service engineers do it for you.  
Each provider will disclose international calling rates on their website and a list of features on their website. We give a standardized list for each provider (with explanations on our VoIP Calling Features page) but providers experiment with different features all the time. Check their website (using a link on their details page) to verify how each  feature works.
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.
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