People who have tried many of the free RTC softphones have observed that they don't always work through firewalls or NAT networks. Sometimes these problems give visual feedback, in the form of error messages advising that the call can't proceed. In other cases, the call appears to be connected but audio only works in one direction or stops after some brief period of time.

Metcalfe's law tells us that the value of a telecommunications network is proportional to the square of the number of connected users of the system (n²), demonstrated in Figure 4.1, “Metcalfe's law”.

Therefore, the benefit of making the solution work for all those users who may suffer in certain NAT environments does not just have a gradual or linear impact, the benefit is quadratic.

Today's RTC technology gives us the tools to deal with these problems in the vast majority of cases. This chapter gives an overview of the main concerns.