240 is a bit of an odd place for ringback. It could definitely still be on there, but what with area codes being filled up so quickly, most phone companies have moved them over to either 958 or 959. This is a program the switch runs though, so it more then likely is still someplace. Since it's indigenous to the switch you're on, what it can do, and where you can find it change from one town to the next.
On a DMS-100, the ringback program will also check your touchtones. Dialing 1234567890, *#, or possibly ABCD if you have a phone with those keys will give you two beeps if everything is okay, and one if it detects an error. Like you mentioned, you can flash for ringback. It'll give you a 480+620 hz tone to tell you to hang up.
A 5ESS is kind of the same deal. Flashing will give you a 480 hz tone and ring your phone. To check your touchtones, dial 1234567890, 4-12345689*0#, or 5-123A456B789C*0#D. Sometimes you'll see slight variations, though. For example, one switch I was on let you dial 2 to stop the dialtone (as opposed to keep it there as you dialed), and then dial 123456789*0#.
On an EWSD...well, I have no idea. Those things are weird. JmanA9 knows how their ringback works, though.
On a GTD-5, you'll get a bunch of short 480 hz beeps with lots of silence between them. Once you dial the code for ringback, I don't think it'll allow anything special, but hanging up will ring your phone back. Alternatively, a lot of these switches are fitted with a proctor test set. Technically, the GTD-5 isn't ringing you with this since it sets up a direct, metallic test path. Quite literally everything comes from the test set, even the power coming to your phone line.