Anatomy of an SIP VoIP Call
Clients and Servers
And SIP VoIP calls hand at the heart of it, two important components. The client and the server. The SIP client is a program that you installed on your device whether it's a PC, a smartphone or a tablet. This program can be provided to you either by your SIP provider or by any third party who has designed the client to work with a standards-based SIP provider. They can be downloaded from a variety of places like the IOS or the Android marketplace and customers have a large variety of choices and can select what kind of look and feel you want.
The other part of the model is the SIP server which is located with your SIP provider. This is the component that does all the heavy lifting and sends data to your smart phone through the SIP client. An incoming VoIP called is processed by the SIP provider through the SIP server and routed through to your client which makes your phone ring. It's really that simple. There are similarities to how we use e-mail with the difference that the SIP VoIP protocol takes place in real time with no facility for "storing" calls other than voicemail.
Pure VoIP and PSTN Calls
There are two types of VoIP calls. One which goes entirely over the Internet between two SIP VoIP providers and is what we call a "pure VoIP call". The second type goes part way over the PSTN network. If you are a VoIP user, you will likely be making and receiving lots of calls which either originate or terminate with a traditional phone user. From your perspective there's no difference between the two. Your SIP client allows you to make and receive calls in conjunction with your provider in exactly the same way regardless of who the recipient is as long as they have a telephone number. At the back end, your SIP provider first checks to see whether the telephone number you dialed belongs to a known person within your own VoIP network. If so, its directly routed to their SIP client over the Internet at no charge. This is why all VoIP calls to other subscribers on the same network are free. It's just like e-mail.
If however the telephone number cannot be sourced to an existing SIP address, your provider passes the call over to the PSTN network. This network resolves the number and routes it in the traditional manner. Selecting SIP providers can be quite confusing since is important to choose one that suits your business needs. Opting for a hosted PBX system instead of your regular telephone line will save you huge sums of money starting almost immediately.