VoIP Gateways and Signalling Switches
It's easy to understand the concept of VoIP. On the face of it, there's very little to explain. We can communicate with each other using systems like email. We can watch videos over the Internet using our unlimited data plan. It's pretty trivial to imagine that we can also send voice signals to each other for little or no cost right? In fact, that's exactly what happens when we want to talk to one another on compatible systems such as Skype and Google Voice.
Of course, real time voice has different requirements from streaming audio/video which have the luxury of buffering before they start playing, but these are technical challenges that have been overcome for the most part. Burgeoning Internet speeds have pretty much ensured that VoIP will never face bandwidth constraints that "plug up the pipes" as long as there are just a few users.
The real issues however, come about when we want to expand the use of VoIP to other networks. It's like trying to send an email to a person without access to the Internet and who can only receive messages through the postal service. Imagine the complexity of that! But if VoIP were to take off, it would certainly have to integrate with the regular PSTN systems both ways. And thus the need for VoIP gateways is born.
SIP and SS7
When two networks such as VoIP and PSTN have to interconnect, one of them has to take the dominant role. Though VoIP is clearly better in many ways than the old telephone system, the penetration and usage of the PSTN systems is far ahead of VoIP. Therefore, many aspects of VoIP have to be modified to suit legacy telephone systems instead of the other way around. The most obvious change is the use of "telephone numbers" to call other people instead of the Internet way of doing things by saying "firstname.lastname@example.org."
A VoIP gateway is a device that allows traffic exchanges between the two networks. The SIP addressing system allows each user to be identified just like an email address using the SMTP protocol. SIP itself is a signalling protocol for VoIP. Most telephone networks in the world use the SS7 protocol for signalling so what we need is a device which can convert the signals back and forth and this is one of the primary jobs of a VoIP gateway. The actual architecture is complicated and beyond the scope of this article.
If you're going with a hosted phone system, none of this will matter to you. All you need to figure out is which hardware to install at your location by going through PBX phone system reviews to determine the IP phone of your choice. The VoIP gateway's technical details will be handled by the ITSP.