Sometimes, entering deep engineering modes ( # #3646633# # ) and resetting the NVRAM accidentally deletes the IMEI. V4.1 is the only low-level fix before resorting to a full re-flash.
Click Write or Start . Follow the on-screen prompts (such as powering off the phone or connecting it via USB) and wait for the "PASS" message. Spreadtrum Imei Tool V4.1