You can’t. You can’t. You can track which network routes are used with specialized sniffer software but this does not determine the final destination or identify the user device.
But then, there’s really nothing like unfixed NPA numbers. Devices may make calls without needing to have a telephone number. But its PBX NPA can still identify the PSTN network that it uses for the outbound phone call. Prepaid mobile phone cards are the clutch thing that can be defined as non-fixed identifiable. Free VoIP providers have a legitimate telephone number and enable the termination of the inbound phone call. But they still need a user ID and account setup. Even if the passwords used are not legitimate and the consumer is lazy, the person using that password can be traced back to. It requires diligent physical and technological work to decide who the customer is. VoIP is a protocol for communication. Only the capacity to transform analog soundwaves into physical tcp / ip internet packets.
It can be routed point to point (i.e. Skype to Skype, or Facebook Messenger to Facebook Messenger, FaceTime to FaceTime, etc.) without a telephone number.
VoIP uses also a native / proprietary codec which is converted to data packets, routed to a PSTN gateway, transformed into an internationally acceptable modulation of the digital pulse code (e.g. G.711), and then, via signalling, transmitted across the overall telephone network.
As described by (countries) routing telephone numbers for each PSTN toll (i.e. class 4), the telephone carrier interface to the global network will turn off a call to a local class 5 tandem switch. Everything operation is recorded by both forms of switches. If the VoIP call is local, it connects only to the closest tandem switches.
Phone numbers can be assigned individually for each telephone or the assigned phone number of the Private Branch Exchange (PBX) (i.e. call center or larger corporate environment).
Telephone numbers can be deliberately hidden (masked) or secretly branded for a service charge, according to telecommunications legislation. Most nations are no longer able to use hidden caller ID (i.e. * 67). Many VoIP systems have the same features.
If you want to link a PBX VoIP phone to the PSTN network and receive direct inbound calls, it has to be recognizable in a VoIP PBX DID folder.
However, this does not mean that the monitoring on outgoing calls is required for any phone call from that network. The mobile service company must be certified as a contract customer, but that’s it.
Most telecommunications authorities mandate all mobile carriers to keep records of all PSTN calls carried for a period of one year.
You have to keep internal call logs (PBX / controller / client when necessary).