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Chapter 12
PSTN Trunks
While private trunks connect voice switches on your private network, PSTN trunks
serve another purpose: connecting your PBX or your VoIP network to the outside
world. They can be analog phone lines, digital phone lines like T1s, ATM connec-
tions, or VoIP based, depending on what kinds of service are available from your
PSTN carrier.
Legacy telephony purists will balk at the use of the word trunk to describe a T1 or an
ATM connection, arguing that a trunk is nothing more than a phone line connecting
two switches. In fact, the definition has grown to mean any connection between two
voice networks. A 5-mile-long T1 between two old-school PBXs is a trunk, and so is
a UDP pathway between two VoIP servers. Even in non-voice scenarios, the word
trunk is used to describe a pathway between two switches—take VLAN trunks as an
The way you think about trunk connections is different when they’re PSTN trunks.
While privately owned trunks are relatively cheap or free, PSTN trunks incur service
fees. Careful design, utilization, and monitoring of PSTN trunks is important to your
bottom line. PSTN trunks can also offer calling features that let you do things that
may be less easy to do with private trunks: features like distinctive ring and three-
way calling can be integrated into your voice network to simplify your PBX design or
to enable functions that you otherwise couldn’t provide.
In this chapter, we’ll cover the fine art of connecting to the phone company: choos-
ing a dial-tone connectivity technology, locating dial-tone trunks, and providing
ample voice bandwidth for next-generation telephony apps. We’ll also cover a vari-
ety of techniques for integrating PSTN trunks into your voice network, including dis-
tinctive ring, automatic call distribution, and find-me-follow-me.

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