I Have a Nokia Lumia 710 and sms - mms - and group messaging does not work.
I place the following setting in the Network setup app :
Under MMS APN
MMS access point name: att.mvno
Proxy address: 66.209.11.33
Proxy port: 80
MMSC address: http://mmsc.cingular.com
Data works fine on the phone.
any one care to help?