I don't see much benefit in the procedure artemis entreri describes; you're essentially making an ad-hoc (and therefore likely leaky) acoustic coupler and then introducing your land line circuit into the mix just to get power and a dial tone for the phone handset, which is overly involved and gains you little relative to simply using the land line directly. The benefit for Kevin Mitnick, of course, was that it takes longer to trace a cell phone call than a land line call, but the rest of us probably don't want to end up like him, do we? So probably you just want to make a call when you have nothing but your cell phone on and, or your cell phone has free long distance, or the like.
Cell phones are cheap these days. Often you can get a second (none-too-fancy) phone with a calling plan for a few dollars extra. Why not simply open up the phone itself? You might need some Torx drivers to get it open, but once you've done that, it's a simple matter of cutting the wires running to the speaker and the microphone and alligator clipping to an RJ-11 connector ripped off a random phone cable (or going whole-hog and buying a connector and crimper from Rat Shack). The dial tone problem is easy to work around with a little knowledge of the Hayes command set: ATX0 tells the modem not to bother listening for a dial tone before dialing. So you tell the modem ATX0, then dial the cell phone by hand, then tell the modem ATD to get it to start negotiation. Ta-da! now u r 1337.