The Public Switched Telephone Network (PSTN) remains pervasive and convenient for much of the world's population, even though it is also expensive, unsophisticated and insecure.

In the early days of IP telephony, the PSTN was perceived as something of a security blanket. Companies would design their IP telephony system to mirror the traditional PBX. Some would even keep a few analog phone lines connected in case of an "emergency".

This fear-based approach has gradually been replaced by a more pragmatic approach, relegating the PSTN services to being just one part of the big picture. A modern RTC-based solution works seamlessly with or without the PSTN.

The emotional attachment to the PSTN has largely been contained due to two factors: the widespread presence of smartphones as an alternative means of communication that can be used in an emergency and the increased investment in corporate IT networks which need to be always available.

This chapter looks at PSTN connectivity issues.