X10 had a controller called a Touch Tone Responder. The model number TR16A and X10Pro had a PHC06.
Basically you called it and when it answered the phone you keyed in a three digit security PIN code. Then used touch tones to turn on and off X10 devices on the House Code it was set to.
It also send an on off signal on the House Code and Unit Code of the third security code dial as the phone rang. I have mine sounding a Chime Module so I know when a call is coming in
There is a time delay switch so an answering machine could also be on the line.
Limits where only Unit Codes 1-10 could be used as 11-16 had no corresponding touch tone.
I have not seen them listed lately and they maybe discontinued.
If you would like to look at the manual to see how it worked.
http://software.x10.com/pub/manuals/tr16a-om.pdfSome X10 dealers may still have them even if X10 doesn't show any on their web site. A Goggle search did find a few that still may have them.