1. Phone Credit Flying Out The Window? Turn 'OFF' EDGE

This is probably the most $$$ expensive problem most new iPhone users will encounter so it ranks #1 in this section. The EDGE connection is 'ON' by default on every iPhone. This will incur hefty charges to your phone bill if the connection is used by applications over long periods. If you actually want to use EDGE regularly though over 3G or Wi-Fi, you may want to look at signing up for unlimited data plans which you can ask your telco provider about.

Most of us don't use EDGE unless we need MMS functionality, so the fix is as follows.

a) For non jailbroken phones, enter any irrelevant text such as "nothing" or "disable" in the APN field (Settings> General> Network> Cellular Data Network> APN) to disable EDGE.
APN: xxxx
APN name: xxxxx
APN password:xxxx

If the above method above does not work, please read: 2. Maxis APN Settings Locked? No Carrier Signal? below for more info.