Answer by Westley Julio:
It really depends on what you’re trying to accomplish.
Almost any SIP device (such as desk phone) can register or otherwise accept/send SIP traffic from or to a network. For example, you can register a phone directly with any number of VoIP providers, without a PBX. You can make and receive calls.
It’s when you start getting into more advanced features that you need a PBX, Session Border Controller or related technologies.
For example, if you wanted to provide Voicemail, IVR, Queuing, Unified Communications, T.38 Fax to email, ring groups, CID based routing, etc. a PBX might be the best way to handle this. Some of these functions may be available on a VoIP provider’s end. However, you need to reconcile costs, the experience you intend to deliver, QoS, etc. factors to decide what route to go.
If you’re intended to provide voice services to an office for general telecom needs your infrastructure, carrier partner and server requirements will be different than delivering virtual receptionist, automated notifications (EG doctor’s appointments), predictive dialing (ugh!), etc. type services.