I think you are confused by the fact that there are two products. One is Jitsi Desktop and the other is Jitsi Meet.
Jitsi Desktop is a desktop application which you install on your PC and use to connect to XMPP or SIP services, the choice of service is yours, we do not offer any service. XMPP is generally used for chat and can be used and for audio video calls. SIP is for telephony where you can call PSTN/mobile numbers if the service allows it, and you manage the payment with the service you have chosen. For example, I had been using ippi.com.
Jitsi Meet is a video conferencing solution. We are hosting a deployment you can try, meet.jit.si, which is the one behind the button “start a call”. You can send a link from your meeting to the other participants and they can join. For that, you need only a supported browser Firefox and preferably Chrome. It allows dialing out to numbers, but as it is a demo service we limit the calls to few minutes and limit the destinations allowed.
Jitsi Meet you can also install, so you can host your own deployment, behind your domain name, but for that you need a server somewhere on the cloud with good bandwidth. This is not supposed to be installed on a user’s desktop PC. If you manage your deployment you can deploy and dialing out to telephony without any restrictions.