What is SIP ?
SIP is a protocol for establishing Voice over IP calls. It is an open standard, published by the IETF organisation, who made the vast majority of internet protocols. It is inspired from e-mail and http. The following section describes the base principles of SIP from the user's standpoint, and how to make SIP calls.
Direct SIP calls in local network
Just enter sip:<hostname or IP address> in the SIP url bar of linphone to place a call to another linphone running in your network.
This works also on the public internet provided that the two machines have public IP addresses or appropriate firewall rules.
Internet calls through a SIP proxy
On the public internet, because of dynamic IP addresses, mobility, firewalls, direct calls are not really usable. That's why the concept of SIP proxy exists: the proxy is responsible for routing calls whatever the IP address of end-users is. VoIP operators company are deploying SIP proxies on the internet to get SIP calls routed for their subscribers.
In a first step, the user has to subscribe for the service by choosing a username (sometimes a number) and password. Your public sip identity is then sip:<your username>@<your voip provider>, this is the address that other people can use to reach you using SIP.
Then, by configuring the client (such as Linphone) to go through the proxy, you get a public sip address on the internet, in the same way you have an email address. Other people subscribed to this operator can be reached simply by typing their username or phone number.
The routing of pure internet calls is a service that is often free. Linphone.org is powering a free SIP proxy service, you can register here.
Calls to normal phone numbers
VoIP operators for most of them also provide attractive prices to place VoIP to PSTN (= classical telephony) through their proxies and gateways. You could then call anybody in the world from your favourite linphone client, by simply entering his/her international phone number. The SIP username becomes a phone number.
Choosing a voip operator
The Linphone project is not affiliated with any VoIP or telecom operator, thus we do not recommend or promote any of them in particular. By typing "voip operator" in your favourite search engine you will get hundreds of answers. You can refine the search by country to get more pertinent answers.
Linphone is using the standard and well known SIP protocol. As such, it should be compatible with any voip operator using SIP for its network.