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?