i think i played with carrier setting

I also got this problem on my iphone 3g.
this how i fix. i use digi.

1. remove sim card on iphone.
2. try use ur simcard on other phone to search network (if u see signal strength)
3. try put back ur simcard on ur iphone.
4. it works for me.

but the problem not stop there.
i still got the problem when i go to place where no digi coverage for a minute.
then its happen again. signal carrier  "network lost". "no service" display.
i have to redo above step again to getting signal digi works.

i'm still searching the best solution.

any help?