When someone dials your number, the call enters our network and is presented as a SIP (Session Initiation Protocol) INVITE — the message that sets up a call and carries the number that was dialled. You receive that INVITE at your PBX (Private Branch Exchange, the phone system that routes calls inside a business), softswitch, softphone, or a plain forwarding destination. You choose how the call is delivered, and every method can name a configurable failover destination so a call still lands if the primary target is unreachable.