Here's what a recently deployed soldier has to say about Marisa's questions. Note - I added a couple of questions of my own that might be of interest to your readers.
First, can a U.S. soldier use his American cell phone in Afghanistan (keeping his American phone number)? I don't think so. The problem is the SIM chips, you have to use a local Afghan cellular network provider and their SIM chips will have an Afghanistan phone number.
Second, do they have AT&T Call Centers (using pre-paid phone cards) at American bases in Afghanistan? I don't remember seeing "AT&T Call Centers" specifically, all of the bases I visited had "Sniper Hill" call centers with both internet and telephones for use at a nominal price per minute. See http://www.sniperhill.net/plans-pricing-personal.php
Third, is there a certain internet service provider that is used by military personnel stationed in Afghanistan? In other words, IPs of which provider would be in email headers of military folks stationed overseas? There are many ISPs with access to the U.S. bases who provide connectivity, it varies by the specific base that you are stationed at but even small combat outposts had network access. The 1-month cost was about $90.00 through Sniper Hill with the other plans being cheaper.
Fourth, can soldiers use webcams for video chats while deployed to Afghanistan? Absolutely! I Skyped with my wife twice a day. I used the on-base ISPs, the connection is not fast enough for on-line gaming, like World of Warcraft, but there was almost never any latency with Skype.
The major gave me very detailed answers to my questions, but I'm not able to reveal everything he said (for security reasons). Just as U.S. soldiers have network access in Afghanistan, so do members of the Taliban. There's no point in telling them the specifics of how our communications systems work. In any case, I hope this was helpful.