iMessage doesn't activate

i use iPhone 4(iOS 7). when i try to activate imessage from my phone, it shows a message about operator sms charge. though it charges for the sms, but my imessage doesn't activate. if imessage button is on, it always charges for 1/2 sms per a day, but imessage doesn't work. point to mention that, it's not a iOS7 problem on my phone. i used to face same problem on iOS 6. how can i activate it? give me a solution plz.............