Have an older system (command line only interface) to which I need to add one phone.
Look at extensions.conf and the channel technology driver configuration file (chan_dahdi.con, iax.conf, sip.conf, pjsip.conf, etc.) and duplicate the device and extension information, from a similar phone.
Asterisk is a tookit, which means one has to see how it has actually been used before one can extend the configuration in a compatible way.
Note that is is possible that the above files contain include lines for the actual files you need to modify.