Have you checked your APN settings?
For AT&T (CC uses AT&T or T-Mobile):
Name reseller
Proxy port Blank or Not Set
Proxy push Enabled or None
Authentication type secure, none, or not set
Username <Blank>
Password <Blank>
APN reseller
MMSC
http://mmsc.mobile.att.net
MMS Proxy proxy.mobile.att.net
MMS Port 80
Authentication Type None or Not Set
APN Type default, mms, supl or
default,mms,agps,supl,fota
For T-Mobile:
Name CC
APN wholesale
Proxy none or not set
Port none or not set
Username <blank>
Password <blank>
Server none or not set
MMSC
http://wholesale.mmsmvno.com/mms/wapenc
MMS Proxy none or not set
MMS Port none or not set
MCC 310
MNC 260
Authentication Type none or not set
APN Type default,supl,mms or
default,mms,agps,supl,fota
Also, do you have a Connect Plan, and do you have any data left on it for the month?