Lua SwxWare v13.10
This function returns the name of the user that transferred the call into current call routing script.
local sName = PBXCall.TransferrerName()
This function returns a string value.
If the call is a direct call and not a transferred call the function returns an empty string.
If you are interested in the number of the user who transferred the call you can use the PBXCall.TransferrerNumber() function.

By Tom Wellige