SIM800: AT-command to get connected sender ids
The command you are looking for is AT+CNETSCAN  See page 197

The datasheet only shows a few of the commands.  The link I gave leads to a complete set of all AT commands for the SIM800.

Excerpt for AT+CNETSCAN:



Note that it tells you the country code, the operator code, and a cell ID code.

You will have to find a database somewhere inorder to resolve that information into a location.

I expect the cell IDs to be country dependent, and probably operator dependant as well.  That means you'd need to get that information from the company that operates a particular cell.

I found this site which will accept the given information and return a location.  I don't know if they have an API you could access in an automated way.