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It is illegal in India to connect VoIP to PSTN lines, but some criminals have done this to carry out illegal trade. Sometimes they are caught by the authorities here and sometimes not. Could you explain how they are tracked down?
Any company that provides a circuit has the legal ability to "snoop" the packets to determine their contents. That is not to say that ISP companies snoop all packets and decode their contents, but where the routing protocols and software allow, the type of packet can be determined. For instance, it is easy to determine which packets are voice and which are data simply by looking at the prioritization level of the packet or by looking at the IP port in which the packet expects to communicate between the end station and the sending device. Newer VoIP technologies set an application level tag inside the packet to help the network determine priority and type of service. Routers can be queried to determine which devices are sending which types of packets simply by running a report on a network management system with that capability. Bandwidth managers are a good example of this type of software.
has a great whitepaper on network prioritization. Being able to throttle back data and prioritize voice is a function of this software. In order to do this, each packet has to be examined. If tracked and stored, a simple report is all that is needed to determine the device and routing path.