What are the main components of the VOIP network?

VOIP Gateway, VOIP Provider, VOIP Server.

But why VOIP Provider is needed.
It can be any Telecom service provide.

Call Agent which is CCM for cisco, then you need Gateway (ISDN GW) to connect to the PSTN If you want a connection to the ITSP then in this case you should use SBC ( CUBE for cisco) Of course there are additional servers for VM (which is CUC for cisco), Presence (CUPS)