I wrote earlier that a simple UA-UA call without any service provider is not used in practice. But do we really need a service provider if we are satisfied with SIP to SIP VoIP calls?
Depending on the UA, no registration is necessary to initiate outgoing calls.
E.g. the Grandstream SIP phone can be configured to not register an account.
Publicly reachable SIP addresses, provider's and PBXs, do not authenticate calling UAs. Calls are possible!
Outgoing calls are possible without any subscription or service provider infrastructure! If no special services like PSTN interworking, voice mail, etc, is needed, no third party is necessary for the outgoing call leg, except standard DNS services.
This scheme is meant for registered phones, the service provider's server has to analyze the number and make routing decisions.
Outgoing calls calls from an unregistered UA with numeric keypad only can be made in following ways:
1) There might be some programmable keys or an integrated phonebook. Here it's possible to store some fixed SIP destinations in alphanumeric form.
2) If mainly one or two destinations, who use internally SIP addresses with numeric prefix are to be called, special account's can be created for those destinations. The Grandstream GXP2020 supports 6 SIP accounts.
3) Some UAs support even enum queries. In this case the full number, usually in international format, would have to be dialed. The enum query would return a SIP address and normal SIP session setup can take place.
Dynamic DNS services allow terminating requests to non-static IP addresses, like behind a DSL. Together with SRV records, SIP requests can be directed to the correct IP and port.
Special attention in this scenario has to be paid to NAT routers. Because there is not provider support and mechanism to get across NAT, port forwarding needs to be configured for each SIP device behind the NAT router.
SIP to SIP calls are possible without any service provider. There are some limits for desktop phones without alphanumeric keypad.
Nevertheless, there are numerous service providers which offer SIP to SIP for free and make dialing more comfortable, especially for desktop phones, plus offer other services.
Providerless SIP phone calls are more of an experimental nature. But this shows that consumers get more independent of traditional telephony service providers. Decentralized services are thinkable.