GSM Data Post Over the GPRS Using GSM Modem
There isn't much information in your question, but I'll explain one common way you can solve this type of problem. In short: You need to get more troubleshooting data, and then use your critical thinking skills to find the cause of the problem by using that data.

Remember that responses like "HTTP 400 Bad Request" come from the HTTP server. Therefore you need to find out why the HTTP server didn't like what it received, when you used the Airtel SIM.

One way you can do that, is to setup your own HTTP server (if the server you are currently using is not already under your control). Then you can look in its access log (or use tcpdump or wireshark etc. which is my preferred option) to capture what the HTTP server is receiving in each case.

The response from the HTTP server was different between your two tests. Therefore it must have received something different when using the two different SIMs. By capturing the HTTP request received at the server when using the Vodafone SIM, then doing the same thing with the Airtel SIM, and analysing / comparing those 2 sets of data, you will find what is causing the HTTP server to respond differently in your 2 tests.