Local calls

It's not unusual for VoIP providers to struggle to provide value for money over a traditional PSTN service for local calls. In many markets there has been keen competition in this area that has resulted in a reduction of charges to the point where many very competitive deals can be found. For customers implementing an Asterisk-based system making many calls to local landlines, the answer frequently is to break out on to the local PSTN immediately rather than route via an ITSP. Of course, the calculation should be made in each case, as at lower volumes the cost of line rental can become significant. For instance, a company that tends to make many local calls during a short period of time (either a small part of every day or during a short period of each week or month) may end up needing a high number of landlines for a relatively small overall number of calls.

For customers where using PSTN for local calls makes sense, there are a couple of options. Probably simplest is to fit one or more PCI cards with either analog or digital interfaces into your Asterisk server(s). Conversion from IP to analog or digital is carried out by the server, increasing the use of its processing resources but removing such traffic from the LAN. Alternatively, PSTN gateway devices exist that sit on the network and interface with analog and/or digital PSTN lines, presenting the traffic to the Asterisk server, usually in the form of SIP trunks. They carry the advantage that they will not fail should the server do so. In this fashion, they can form an important part of resilient solutions.

Integrating Asterisk with analog and digital PSTN lines is discussed in detail in Chapter 9.