Some AT Commands does not response on SIM800L (gprs activation)

Hello

I'm using Arduino uno and SIM800l EVB Modules.
I have little problem in AT Commands.

I can send and receive sms massages.

i'm trying to activate gprs functions, when i'm sending just "AT" it comes OK but when i'm sending AT+CFUN or similar commands it comes error.

Does somebody knows the reason of it?