However why don't ATT pple tell me the same thing?
Because they simply don't know. If you were around these boards at all in the few weeks before the 3G iPhone launch, think back to all the posts by people wanting to upgrade, stating they had called AT&T and been told this, told that, told something else, mostly conflicting information, mostly wrong. First off, unless you enable international roaming with AT&T, you won't even be able to use your iPhone overseas (except as an iPod), so you won't incur charges at all.
You should read AT&T's web page on international travel with the iPhone:
http://www.wireless.att.com/learn/international/roaming/iphone-travel-tips.jsp
Also, the following is from the iPhone Manual:
+*Using iPhone Abroad*+
+You can use iPhone to make calls in many countries around the world. You must first+
+enable your carrier’s service plan for international roaming.+
+Enable international roaming: Contact your carrier.+
+Set iPhone to add the correct prefix when dialing from another country: In Settings,+
+tap Phone, then turn International Assist on. This lets you make calls to your home+
+country using the numbers in your contacts and favorites, without having to add a+
+prefix or your country code.1+
+Set the carrier to use: In Settings, tap Carrier, then select the carrier you prefer. This+
+option is available only when you’re traveling outside your carrier’s network. You can+
+make calls only on carriers that have roaming agreements with your iPhone service+
+provider. For more information, see “Carrier” on page 101.+
+Important: Voice and data roaming charges may apply. *_To avoid data roaming charges,*_+
+_*turn Data Roaming off_*.+
+Turn Data Roaming on or off: In Settings, choose General > Network and turn Data+
+Roaming on or off. Data Roaming is turned off by default.+
Note that visual voicemail
does use data (and if you are outside your carrier's area it will not work with data roaming turned off).