A few questions about IP telephony networks?

Question

Telephony

Specifically interested in the operation of the network in the picture. What happens if you connect the phone directly to the Ethernet from the provider? How are own numbers assigned like DNS? That is, by connecting the network as in the picture, it will be possible to call the office from anywhere with Internet access? But how to call from an ordinary telephone network, and is it possible to contact a network connected according to the topology in the picture (and by what number in general) from a regular phone? And how to integrate specifically these Ubiquiti IP phones into a regular telephone network, if any - through any VoIP gateway connected to the switch? And is it necessary to notify the provider / PBX, or will everything work by itself ?
I understand that the questions are very stupid, but after reading a lot of information, I still don’t understand how it all works in practice.

Answer 1

So let's go in order.
1. In the picture, the phones are not included in the provider's network, but are included in the local network, in a separate port of the router, which is already responsible for issuing IP addresses for them.
2. Judging by the fact that there is no PBX in this scheme, it means that the phones are connected to the cloud provider. Those. There is always a PBX, just in this case, for simplicity, it is not indicated.
3. Once there is a PBX (and it definitely is), phones are connected to it (it can be local or somewhere. It can support SIP \ H323 devices directly or through VoIP gateways) and it is already responsible for call routing, for communication with the city PBX, for landing incoming lines (again - by stream directly or via SIP).
4. Since the numbers are landed on the PBX, then you can call them in the same way as a regular number. The CO will route the call to this PBX, which in turn will forward it to the appropriate telephone set registered in the system or to the group. The device will ring.
5. If you have a PBX in your control, then configure it to register new devices and that's it. If there is no VoIP support, put the gateway on analog / digital lines and already connect the devices to the gateway. If you do not control the PBX, then in any case you need to configure it. But the provider of your external lines does not care who and how you terminate them. For ease of understanding, for your city PBX, all phones are, as it were, behind NAT, and what you do with them outside of this NAT is not important to him.