if they are the approved apple/iphone provider, than you cannot amend the APN/MMS setting. It is already set in the carrier setting. Like Digi and Maxis, you can only on/off the 3G and cellular data tabs in the network setting. There is no cellular data network tab.

However if the provider is not an apple/iphone approved, than there will be a CELLULAR DATA NETWORK tab for you to input the username and password for the APN/MMS settings.

For example, if you put the Celcom simcard (not an APPLE approved provider) to your iphone, the cellular data tab will automatically appear under the network setting where you will be able to input the username and password for the APN and MMS.

The same goes to other provider (not APPLE approved) overseas.