I recently bought a tablet PC, from a Chinese manufacturer.
Its IMEI is 315589840117903. It's showing invalid in India. Please check with this link:
How I can solve this problem? I am unable to connect to internet and unable to make any calls.
The check digit of the IMEI is invalid, and usually the IMEI cannot be changed, or in come countries it can even be illegal.
There are ways to recover an IMEI on Android devices, but for this you'd need a backup of some files from when the IMEI wasn't invalid.