I had a similar problem using the ATT Samsung Galaxy S2 in my 2012 Subaru WRX. Turned out that the problem was when the phone went idle during the call and the screen turned off (to save power). That's when the call would drop. If I kept touching the screen every few minutes to prevent it from turning off, the call would never drop. I solved this problem by simply connecting the phone to a power supply or by checking "never" in the phone's setting when it asked when to shut off the screen.