I had this same problem with a motorola razr some time back. Some cell phones have a built in switch (such as the Razr) inside the external antenna connection. In the case with my razr, I
evidently pushed the adapter into the phone too far. I later had to jump out the switch inside the
phone to make it work with it's internal antenna again. Unfortunately, I could'nt use that same phone with my amp again. Just a thought on your problem.