VOIP is generally referred to when you receive a call from the traditional PSTN on a router that digitizes and packetizes those signals and uses one of the protocols (SIP, H323,) to initiate an IP call leg to the remote device which could be a CallManager or another Terminating Gateway. The TGW reverses the process and sends the call to an anlog port or a digital port connected to phone system. A simple VOIP network would look like
PBX-----Teleph. call leg ---Router-----IP call leg----Router---TeleCallLeg--PBX
In an IP telephony network you can have one Central Call Agent such as
CallManager that controls all local and remote destinations. A simple
topology of a IP Ptelephony would be:
IP Phones-------CCM----Gateway----WAN-----Gateway-----IP Phones
The gateway in this are not doing any voip but are providing a connection
so all phoines can register to the CCM and CCM would direct all the calls, local as well as across wan.
Now in reality voip and ip telephony coexist and there are many variations of that.
I hope that helps.
