Cupertino, CA, United States (AHN) - Customers looking to preorder the iPhone 4 on Tuesday faced several challenges with AT&T's upgrade site going down for several hours, while Apple's online store prevented buyers from placing an order.

According to recent reports, the problem lies with AT&T's site and server, which Apple needs to connect to in order to check a customer's eligibility for the upgrade to the iPhone 4.

The crux of the problem seems to be with AT&T's site when it checks to see if the customer is eligible for the subsidized price of $199 for the 16 gigabyte or $299 for the 32 gigabyte.

Customers who went directly to the AT&T store to preorder also ran into issues, with one potential buyer saying computers were down at the store as well, according to a computerworld report.

Additionally, there have been reports that the pre-order site issues have also led to a security breach, with at least three cases being accounted for, in which a customer logging into their AT&T account to order the iPhone 4 upgrade have been directed to another users account.

Such an occurrence would expose account information, such as addresses, bills and any other private information contained on the site.

AT&T's account system was back online as of 3:30 p.m., ET, but the iPhone upgrade eligibility page remains down.