Since SIP became the commonly used protocol to handle phone calls and other communication and Internet connects almost all households with a reasonable QoS, I ask myself: What is actually the role of the traditional telephone network operators? Everyone can become SIP service provider! Servers and bandwidth are cheap.
Well they have one important asset: Well known and globally coordinated and routed E.164 telephone numbers plus interconnection between all telephony exchanges.
The traditional voice service operators could close their business immediately, if everyone would use Skype and Skype terminals would be used and available everywhere. Or if everyone used Google voice. Or if everyone used "Line", or xxx, or yyy. And that's the point. There are many alternatives and they all are islands. To be reachable I would have to maintain at least a handful of user IDs and different clients.
SIP seems to emerge as the single most used protocol to handle voice and other sessions over IP. There are plenty of clients, some mobile phones have build-in SIP clients and SIP desktop phones became cheap as well. SIP addressing allows email like URIs as well as using the traditional E.164 phone numbers. The question remains who will be the "man in the middle" and operates the SIP infrastructure? In theory this man in the middle would not even be needed for simple UA to UA calls. The user agents can find each other using the DNS. But in practise we want to have voice mail, terminal portability, need transcoding, want to conference, and more.
So plenty of SIP service providers pooped up. They don't need much, some servers and Internet connectivity. They don't have to worry about the last mile. But they want to make money, of course their effort must be paid. They live from the fragmentation of voice services. IP to IP calls are free, but calls to PSTN will be charged, naturally. As more and more users are going IP, the offer of free IP to IP calls can not be kept any more. Earlier many SIP service providers offered free inter-provider calls and allowed incoming and outgoing SIP calls. As the share of paid PSTN calls goes down, those operators become more restrictive. Actually with many there is no big difference if you use them or a traditional PSTN operator. Only the last mile is IP and they are a bit cheaper for this. Otherwise, normal PSTN E.164 numbers are used and inter-operator calls are charged, no matter if they go via PSTN or are pure IP.
Fair enough that SIP service providers earn money with their services. But I wanted to unleash the full potential if the Internet and SIP without any restrictions and free of charge!
I described the basic set-up here.
Although the enum check should happen in the PSTN exchange or be done by the SIP proxy (or IMS S-CSCF), some SIP clients do this already themselves!