We have the Panasonic KX-TG7745, the newest incarnation of this Link to Cell device. Same problem as the OP, although in our case, we have 2 iPhone 4s in the home and one iPhone 4. The signal is perfectly clear on the iPhone 4, no issues. On the 4S's, however, the caller almost always says I sound garbled and typically hangs up because they can't understand me at all. It doesn't matter if I initate the call or answer it....just get that same "Cant understand a word you are saying." I've called an outside line and left myself a lengthy voice message so I could check it myself and sure enough its almost unitelligible.
Using the same recording trick, however, I've also done this at times wher I've had ZERO voice garbling. The 4S was sitting in the same spot (on its charger), nothing else in the house changed (no lights on/off, microwave running, AC/Heater running...everything the same).
So it seems to be a software or environmental glitch. I can't isolate it and have been trying to do so now for weeks to no avail. Anxious to find a fix or I'll have to resort back to a normal phone system.