I have same phone, try this:
APN: att.mvno
Proxy: not set
Port: 80
Username, password, server: not set
Mmsc: https://mmsc.cingular.com
Mmsc proxy: wireless.cingular.com
Mms port: 80
Mcc: 310
Mnc: 410
Ip version: ipv4
Authentication type and apn type: Not set.
Good luck!