Unhappy Signal problem

Hi guys, i am new to android phones and i have a i9000 but i have 2 problems. First, although my signal appears to be full ( 3-4 bars ), sometimes i can't hear another person on the phone, or that person can't hear me...it's like i'm having bad reception. The second problem is that sometimes when somebody calls me, the phone doesn't ring and doesn't turn up...nothing happens, i don't even know that i was called.

Firmware version 2.3.4
Baseband XXJVR
Kernel 2.6.35.7-I9000-CL425308

Thanks.