I have a p900 and it was exactly the same problem: the phone worked only when he want, and has full signal but gave 3 beeps. Only 1800 gsm frecvency has this problem. In other networks it work greate. I have used the phone for about 2 mounts or maby more on 1900 Orange frecvency. After that I tried again to 1800 cosmote to see that the problem has resolved