Called them and found out it was an APN issue. (Yup. I have a SIM adaptor for the Nano. I ordered a Nano so I could test the iPhone and G4 together.)
When I first stuck the SIM card in, the only phone number I could call was 611. After setting up the APN, I can now use the phone and surf, so...
In case anyone else has an issue, here's what you need to do to fix it:
Settings/Mobile data/Menu(Top 3 dots)/Mobile Networks/Access point names/Menu again/New APN
Name: ATT Phone
APN: phone
Proxy: Not Set
Port: Not Set
Username: Not Set
Password: Not Set
MMC: http:\\mmsc.mobile.att.net
MMS Proxy: proxy.mobile.att.net
MMS port: 80
MCC: 310
MNC: 410
Authentication Type: None
APN Type: default,mms,fupi,hipri
APN protocol: IPv4
APN roaming protocol: IPv4
Save, then select.
Lastly Reboot.
After the reboot, it should be working now.