It's called like so:

{"jsonrpc":"2.0","method":"call", "params":[3, "get_followers", ["jesta","",1]],"id":0}

The 3 in the call might vary from server to server as well, as it's the API number to use. You can get the ID by running:

{"jsonrpc":"2.0","method":"call", "params":[1, "get_api_by_name", ["follow_api"]],"id":0}