Did you experience call drop off at start of the conversation or after some time during the call, with you android smart phone?.
I had experienced similar issue with call drops off during conversation with an android smart phone. I have tried few steps to get it fixed.
There could be many reasons behind call drops off while in conversation.
One may be due to the network. If you are in call and your current position may be in the border of two cells. So, due to the change in signal strength your call may get routed through a neighboring cell, but there is possibility that the new cell through which the call is getting routed is already serving its maximum capacity. Then your call may get dropped!. In order to fix this, always makes sure that you are at a position where your signal strength is maximum.
Another reason could be at the handset level. In android I had experienced the call drop few times. Then I tried to trace out the root cause for this behavior. I could find that,,I started experiencing this issue after installing few android apps.
When I tried to analyze the apps installed, I found that there was a call filtering app installed from android market. I suspected this could be a possible reason, because call filtering would be directly dealing with call controlling drivers. I had removed the call filtering app and restarted my android phone. The call drops off issue got resolved!
You may try the same steps if you are facing the call drops issue with your android phone. I also feel same approach can be applied other smart phones also.
If you could find some other solution also, please feel free to comment here.