M10 can not transmit data through tcp/ip after successful connection to the remote server

We are using M10 modules in our GPRS-modems. We can successfully connect M10 to the remote server and send data through the tcp/ip with the most of gsm service providers. But with some SIM-cards of russian gsm service provider “МТS” we have a problem. Although M10 successfully connects to the remote server with these SIM-cards but the data is not delivered though AT+QISEND returns SEND OK status.
We can not find out any reasons of failures of starting data transmitting after successful connection as well as we can not understand why in some rare cases transmitting starts successfully and lasts until we reset M10.
Can anybody help?

Sorry for the late reply.
What does AT+QISACK return when you have sent the data?
The return value has three arguments, and if the last one is 0 then you need to grab the log and send it to us for analysis.

Thanks for reply!
Recently i found out that the GSM provider can detect the type of the device (modem or phone) looking at its IMEI number. Then provider can block tcp data transmission for modem, according with the tariff!
But i still not get the confirmation of this fact from my provider so it will be nice if you could confirm such case.