I saw and tried more than 10 ways to repair the IMEI, including rooting the phone, connection to PC, mobile terminal emulator, but nothing works.
The phone is 2GB RAM Lenovo S860. It has 2 sim card slots (I'm using both) 3G and 2G respectively. (I cannot post link because of low reputation, gsmarena has specifications for my phone)
I accidentally deleted the system folder, after which the phone bricked, but I managed to repair it using the following android verion (http://www.needrom.com/download/rom-stock-4-4/) as an upgrade to (http://www.needrom.com/download/lenovo-s860/), both installed using PC.
Now I have the IMEI problem and I really need quick fix because I need the phone for work.