That reminds me. In our student building there was an intercom on each building, and 7 or 8 students would share each apartment. I noticed that it would quietly

That reminds me. In our student building there was an intercom on each building, and 7 or 8 students would share each apartment. I noticed that it would quietly dial the internal number for that apartment. Sure enough, dialing 9 with a DTMF tone generator would get an outside line :)