My iPhone 3GS cant make or receive calls?

I had my iPhone 3GS for 3 months and it worked fine but now it can't receive or make calls. It always fail! I can send texts and go on my internet but can't call. I tried readying my phone even the network. I updated my iphone. Its IOS 4.3.5. It worked fine after I updated it but then the next day it stopped working. I did the hold both keys. I did the settings>general>date and time thing, blow my sims card. But I didn't restore it on my computer. Also I always have 4-5 bars. Unlessni turn on my 3G then I have like 1-2 bars and then it starts searching. Uhh how do I fix it. Thank you.