Why doesn't IP PBX need destinantion NAT?

Question

Telephony

Why, when I connect the PBX to the router (to Mikrotik at the moment, but the same applies to the usual home D-Link) and do not configure port forwarding to access the local PBX from outside, do outside calls go through normally? Am I not obliged to do "destination NAT" (in Mikrotik's terminology, i.e. redirect requests from the external IP address of the router to the internal IP address of the ATS for SIP port 5060 and RDP ports 9000-20000)? By the way, I'm not the admin of this ATSki, but they gave me its config.

Details:
When I took a Mikrotik router and tried to connect an IP phone from the 192.168.2.0 network (PBX address is 192.168.1.1). Calls from the outside went through to him, but no voice was heard, and the call itself was interrupted after 16 seconds.
After prescribing the rules in the firewall according to the examplehttps://www.3cx.com/docs/mikrotik-firewall-configu... calls improved. Unlike the article, I left SIP ALG enabled, it does not work with it disabled.
Why without these rules, with a clean NAT table (for example, with a D-Link router without port forwarding), calls from outside to phones located on the 192.168.1.0 network work fine?

The second question: how is it that despite changing the external IP address of the router to an address from another provider, IP telephony still works without any changes in the settings? Suppose a person dials my local PBX, how does the provider's PBX know where to send packets?

Answer 1

The PBX registers with the provider and says at what address it lives, the login and password have not changed)
> Why without these rules, with a clean NAT table (for example, with a D-Link router without port forwarding)
Mikrotik is not in the default config, damn it what is set there.
It must be remembered that the call consists of two parts, SDP and RTP.
In short.
Your PBX sends a registration request.
The provider confirms the registration and remembers the IP from which the registration came.
An incoming call comes to the provider, it forwards it to your IP .
There is an exchange of SDP packets between the provider and your PBX. The
router sees these packets and automatically opens the necessary ports for the duration of the call.