Heres an idea: forget the coupler or extension cord and all of that. Take a spare male-male rj11 cord and clip and strip one end. attach aligator clips to one end. Put this in your car. When you feel the need to phreak, steal a phone from a lobby of some establishment and plug your pre-made cable into the modular plug. Clip and Call. This eliminates bulk in the cable.
It will look like:
rj11 on phone-------clips
versus
rj11 on phone-----coupler/extension----clips
for those who still dont get it: you make a seperate cable and plug it in when you need it, just like the coupler idea, but far more convenient and low profile, let alone easier, and will work on any phone with a modular female jack.