We probably have six sets of them and use the hell out of 'em 4-5 times a year in our big satellite truck. I think they came from Markertek.
Good example of usage: sporting events. Truck's parked in SNG/ENG truck "pen" and you have 100 feet of audio/video cable pulled from truck to breakout box connecting you to the field, pressbox, whatever.
You ordered hard line phones and the drops are at the breakout panel (common). Or you HAVEN'T ordered hard lines, cell phones are dropping out all over the place and you've managed to bum a couple of spare phone lines from the engineer-in-charge of the ESPN truck. You (a) use the two spare audio lines in your cable run to the box or (b) pull a second siamese line over to the phone drops or to the production truck, slap on your RJ-11 to XLR adapters, hook into the phones, then do the same on your truck's end of the cable to plug into your truck's I/O panel.
Bottom line: you carry hundreds of feet of xlr audio cable on a sat truck. Best to carry adapters (that fit in the side pockets of a Domke bag) and use it as a phone line in a pinch, instead of having to carry a zillion feet of phone line for those 4-5 times a year when you need to extend phone lines.