The Public Switched Telephone Network provides connectivity to legacy telephony systems such as switched long-distance carriers , residential telephony subscribers, and TDM-based business phone systems. Connections between a private voice system and the PSTN are called PSTN trunks or dial-tone trunks
POTS, Centrex, and T1/PRI are the most common technologies for PSTN trunking. VoIP-based links, ATM, television cable, and fiber links are higher-capacity trunk technologies that are supported by some telephone companies
TSPs are companies that provide VoIP-based dial-tone services but don't own the network infrastructure on the "last mile."
While VoIP can help you reduce the number of PSTN trunks you need, there are certain applications, like cell phone bridging, that increase the number of trunks you need
Optical carrier (OC) links offer vast amounts of calling capacity for extremely high-density applications
PSTN trunks are connected to the softPBX in one of two wayseither through gateway devices or through interface hardware contained in the server chassis
The PSTN can be used to link multi-PBX systems via DTMF signaling so that dedicated, sometimes costly, private trunks aren't needed
Asterisk permits you to control caller ID signals for better integration of PSTN-connected switches in the same organization
Automatic call distribution (ACD) is a technique that employs a logical algorithm or interactive data-collection routine (like an IVR app) to determine who in the organization should receive a call from the general public.
Carefully manage the timing of any changes to your PSTN connect point, and do all changes during off-hours.