The old P+ and PIIs were remotely programed using RAUs (Remote Administration Units). One was placed between the set and wall jack at extension 10 and another between a display set and a regular phone line at the programmer's location. The programmer would call the site and take virtual control of the remote set at extension 10.
The connection between RAUs was tenuous, as best. Noise would be interpreted as program commands. Dropped connections and regular failures of the RAUs made them nearly worthless.
I've got two in a box behind my desk here. Great idea, but way outclassed by the ACS remote capability. Before spending a dime on RAUs, use your cash to upgrade the remote CPU.