How and why?
Technically, it is easier to execute call recording in IP. Packets are easily reproduced and destination addresses substituted. In fact, in most IP communications today, such as email transmissions, web surfing, duplicate packets happen quite regularly.
It is the endpoints of the IP network that have the intelligence to order the packets, ignore duplicates (a duplicate packet is recognized as a packet that arrived late through a defective or slower path). This is the distinguishing factor separating the legacy digital framework of Time Division Multiplexing implementations.
In digital networks, the endpoint is quite dumb. It's not a simple matter to duplicate bits and deliver them to a central repository, or to store them, for example. The PBX network must duplicate network resources all the way from the endpoint to the storage facility or device. This weakness leads to cost, complexity and a high threshold of implementation.
Packet networks and IP applications are far more flexible than the digital environment.
In 3Com NBX IP Telephony systems, it is a simple matter to add-in support for call recording. The administration panel has a button to toggle the service and establish the service to an appropriate server product from Red Box, or Esna Technologies for example. Larger enterprise deployments using the SIP-based 3Com VCX IP Telephony module can select their applications partners from a lengthy list including Verint, one of the industry leaders in large-scale enterprise call recording.