I used to have many dropped calls with R9A036 when on the Cingular (or should I say new AT&T) network. It would not happen when I manually switched to the towers they share with TMob or when roaming with Telcel Mexico, even If I talked for hours, but it would't last for more that 5 minutes on the Home network which is AT&T. It would fade out and eventually drop (a few beeps on the phone). I think it has to do with thier ENS or Enhanced Network Selection, something implemented with the merge of companies. After flashing with R4EA031, the problem went away. No more dropped calls.