Anyone knows what call blocker can I use on the 3GS? I was using MCleaner on my 3G but since we can't JB the 3GS, what alternatives do I have?

Maybe you can state which iPhone version (Maxis, SG set, etc), what's your OS version, your operator, etc.

I have a quick question. If I sell my iPhone to someone else, can that person register a new Apple ID and use with the phone? I'm just worried that the phone's serial number is always tied to my email (Apple ID).

Even if you jailbreak, you don't need to tweak anything to enable data usage. Just turn on from the Settings menu like how you do it normally.

Did you try in different areas? Sometimes you can get the 3G signal but there's no connection to the internet. It's just like you being able to capture Wifi signal but that's only from the point of your device to the Wifi access point. What happens from the Wifi access point to outside world (Internet) is a different case.

I've experienced cases where I can see 3G signal but I can't surf the net or check my mails, etc. The reason is because there were network disruptions after calling Maxis. Unless you're saying everywhere you go is the same then that's a different story. Also, try using EDGE to see if it works.