There are two types of conferences that can be built via the Conference module: Local and Standalone.
Local conferences represent the “one-to-many” principle: one incoming call initiates a conference, then our cloud calls up to 49 participants and adds them to the conference. There cannot be any incoming calls to that conference except for the initial one. Local conferences are often used in scenarios when you need to add additional participants on-the-fly during the call, such as adding a supervisor call for call whisper in a contact center scenario.
Standalone conferences represent the “many-to-many” principle: it takes up to 100 endpoints (incoming and outgoing). A conference scenario processes inbound calls from the gateway and initiates outbound calls; all the calls are eventually connected to the conference. Note that if you want incoming PSTN/SIP calls, the conference will require an additional gateway scenario to accept incoming calls and forward them to a conference.