IMEI is short for International Mobile Equipment Identity and is a unique number given to every single mobile phone, typically found behind the battery. IMEI numbers of cellular phones connected to a GSM network are stored in a database (EIR - Equipment Identity Register) containing all valid mobile phone equipment.
Invalid IMEI may cause NO Signal or Unable to Connect with Network Provider.
In some cases IMEI is accidentally delete on factory reset.
Android Invalid IMEI Troubleshooting Guide
1. First, dial *#*#84666364#*#*
2. Go to CONNECTIVITY, RADIO INFORMATION then CDS INFORMATION
3. Phone 1 is IMEI 1 or SIM 1
Phone 2 is IMEI 2 or SIM 2
4. Tap PHONE 1 then type
Note: You should place the IMEI inside quotation marks ("xxxxxxxxxxxxxxx")
Tap PHONE 2 then type
5. Before sending the command check your space there is a space here AT SPACE +!! now Send at Command.
6. Reboot the phone.
7. Check IMEI by dialing *#06#, IMEI should be restored and your phone is ready to use!
NOTE: Engineering commands like AT+EGMR=1,7,"" AT+EGMR=1,10,"" and AT+EGMR=1,11,"" should be available on the dropdown. You don't need to type them manually, just the IMEI inside the quotation marks.