The crowd-sourced coverage maps on Sensorly.com show pretty good 3G service around most of Killeen, so other customers seem to be getting good service there. Have you driven around the area and found generally poor service or is this just in one spot? BTW, you don't need lots of bars to have good service. In my home the signal fluctuates between one and three bars and I never drop or miss calls because of it. However, if the signal is so low that you do drop calls, there is a problem. The SIM card exchange is a placebo. It does nothing to improve your signal.
I will make one observation. It seems that the majority of people who come here with complaints about poor signal level and dropped/missed calls are also iPhone users. That's not based on any scientific survey, just my personal feeling.
If your complaint is about a specific area, the folks from T-Force may be able to help when they show up.