iphone 4s keep missing call

HI

over the last two weeks, my Iphone 4s can't recevie incoming call. I will recieve some missing call text message. This happened one or twice per day. All happened in the similar area within 1 mile. I have been with the same phone network for few years, i don't have any problem with my old phone, such IPhone 3gs, nokia n95, etc. 😕

I went to my network provider yesterday and they said it could be the problem with the microsim. Then they gave me the new sim. But today, issue happened again. Do you guy have any similar issue?

I am ugrading to the new IOS 5.0.1, hope this can reslove my issue.