Can IMEI number of a device really be changed
IMEI is an baseband identifier so it's stored on non-writable memory. There are two IMEIs actually - display and hardware. It's possible to change the display IMEI on rooted devices, but no way to write it down to hardware. Call your carrier for resetting your SIM card and also try to flash stock firmware using factory tools (Qualcomm QFIL in your case)