Asterisk has a number of optional build dependencies.  If you intend to
use any analogue PSTN hardware such as DigiumTDM400P, you should install
the zaptel software and drivers before compiling Asterisk.  If you will
instead use digital telephony hardware (not IP phones) such as T1/E1
lines or a PRI, you'll need to install libpri, again before compiling
Asterisk.
