Doesn’t look like there was really a good answer for this question so I’m posting this 8 months later. The following command will reset back to the factory IMEI number if it is lost or forgotten.
You can restore the factory settings with:
AT+QPRTPARA=3
Don’t use too often because it uses intensively Flash and the module at the end can be destroyed
After you execute AT+QPRTPARA=3, ALL configuration will be erased and overwritten with Factory setting values.