A rotary telephone actually works by "listening" to the number of clicks the dial makes as it goes back around to the original position. For example, a single click would signify the number one, two clicks the number two, and so on ending at ten clicks for a zero. Thus, if there were some other method of making the clicks, the phone could be dialed in an alternate way.
Granted, this is not the most efficient way to dial a telephone number (not to mention very inaccurate), but entertaining to attempt to do when bored. Pressing the disconnecting switch in rapid succession has the same effect as turning the dial to the desired number. Any numbers between one and five are relatively easy to dial. They are short enough to keep track of how many times the plunger has been pressed, whereas a number like eight can leave you wondering exactly how many times the plunger has been pressed, and whether or not it has been pressed fast enough. A pause of too great a time length can result in the phone thinking you dialed a "6" and a "2" when you really meant to dial "8".
This can be done with almost any telephone, not just a rotary, just as long as it has a switch that can be change from tone to pulse. Be sure that it is switched to pulse mode and that’s all there is to it. I recommend starting out calling people with telephone numbers with small digits.
Note: My math teacher was the one who introduced me, as well as my entire class, to this idea. While he was in college, his roommate threw their phone against the wall, thereby breaking the "9" key. Any time they wanted to call someone with a nine in their phone number, they were forced to use the above mentioned process. He gave us a little demonstration in class that proved he was quite proficient at this task.