A client brough me a BLU Studio 5.5C (D690U) which had no IMEI because some other person flashed it. I initially thought of re-flashing it but then decided to do some searching on the Internet and came across this article from http://www.blogtechtips.com/ (http://www.blogtechtips.com/2015/10/05/how-to-repair-imei-number-in-android/).
