I faced the same problem too. Starting from beginning of June, I always can't send and receive iMessage under 3G network of Smartone. After I had tested several days (reboot iPhone / hard reset iPhone / restart iMessage / reset network), I found that iMessage will be malfunction under a weak 3G signal first. Then, iMessage cannot be used even after the signal was improved and only SMS can be sent. If you want to use iMessage again, you can reset the network under a good cellular network or find someone to send iMessage to you when you are in WIFI network. However, problem can only be solved temporary.
I think that this is the problem of Smartone because my friend using Smartone network also faced the same problem recently. Apple iMessage is a good app because it can be used across iPhone, iPad and Mac. Photos sent from iMessage can also be kept in good quality. I believe that Apple will review its co-operation with Smartone if such problem is widespread and cannot be improved 😉