I can't log into my account on my phone

I got switched from T-Mobile to AT&T at a kiosk in the mall 3 days ago because AT&T, which used to have no coverage by my house,now covers the entire county with 3G and HSPA+ 4G, T-Mobile's call quality has never been very good (I switched from Verizon to them a few years ago because my feature phone broke and I got a smartphone, and T-Mobile were the only carrier with decent data deals and no obscenely high overage charges at the time), and my new bill is slightly lower. However, the guy who sold me the plan didn't give me any sort of papers with billing and account information. Texts and emails I got from AT&T after switching did include my account number and ID, but nothing else. I've mostly managed to work around that so far, but I've hit a wall now.

In the MyAT&T app, I entered my username and password and logged in, but it always gives the error message "A temporary system error prevents us from retrieving your account information right now. Please try again later. Further help call 1-800-331-0500.(RESLGN0101)" I know I used the correct username and password because it successfully authenticated, but the app won't do anything. I already had a Directv account, so I've been using that to log in. It's been doing this ever since I switched to AT&T. I've tried rebooting, clearing the app's data and cache, reinstalling all of the AT&T apps, deleting the dalvik cache in recovery mode, and it still doesn't work. The AT&T website won't let me log in at all, except for the forum account I just now created.

Also, I tried to set up the voicemail app, but it wants my PIN. I don't know my pin because as I said, the guy at the kiosk gave me nothing but a SIM card. Is there any way I could get that, or change it if I can't? Would it be the same as the PIN for my account from my old carrier, if I could find that?