Having trouble getting consistent service on my Galaxy S3. I bought this phone a couple weeks ago and I notice my service bounces around from 4 bars to 1 bar then 0 bars then back to 4. It's all over the place. It does this all day and a lot of the time when people try to call me it goes straight to my voicemail - my phone doesn't even ring. I live in a city so I should have really good service. Any ideas how to fix this? Thanks in advance.