In latest firmware, Telnet option has been removed or disabled by Airtel,
So it won't work,
So we need to access the modems internal serial port and need to flash the old firmware using usb to serial controler.
Old firmware has telnet enabled,
After flashing the old firmware then we have to follow this step.
I have done like this and its working.
Now i am using Airtel zte modem with my railwire broadband.