I am having a big problem with my iPhone 3GS. I bought it about a month ago and ever since I have had a weak singal and have been having problems getting some calls. I get a weaker singal than I should be more importantly I will get 1 or 2 bars in the same place I had 5 bars the previous day. A bigger issue I am having is that I don't get a good portion of my calls. I have service but will never get the call. I will see a voicemail pop up without seeing any missed calls. I went to the Apple store and they exchanged the phone but I am still having issues. What should I do next? Should I get the SIM card replaced?