Whats wrong with my Samsung Rant?

[Rawwr. :)]

I just got it Friday. And in the town I live in it always roaming. When I am roaming, It takes FOREVERRR to recieve texts unless Im getting REALLY good service..and *MOST OF THE TIME* when people call me, it wont ring, and wont even show I'm getting a call. But when I'm not roaming it works perfect. I took it back yesterday and they did something to it and said if it still doesn't work bring it back. Is there anyway I can fix it w/o having to take it back? Oh and dont tell me to switch carriers please, Had Alltel it sucks. And My parents owe Nextel money. So plz help