Do you have two (or more) celfones? Which networks are you connected to?

I mean, personally I have a celfone which is connected to ????? and another one which is SUN. And that's my complaint. It used to be SUN's service is "somewhat" decent enough to consider it but the voice call quality really sucks the last few weeks. I really love using my ????? since it gives me crystal clear voice calls and quick SMS send receive capability. Costwise, it is hard to beat SUN but I tend to use it less and less often for voice calls because:

- bad voice call quality
- always gets cut off during the call
- service is busy
- no signal or very weak signal in that area

I just cannot understand why SUN cannot achieve the same voice call quality as other providers.