|You must tell the compiler where to find the function typically by adding the appropriate include directive... or you somehow have to declare the function somewhere.
Is CallUnmanaged in another DLL? Is so, you have to properly export/import the function. If the function is in the same DLL then a simple declaration will do.
Typically function are declared in header files and header files then be included to use that function. If the function is uses across DLL, then it must be exported/imported as otherwise it would be visible from outside the original DLL.
Importing manged code is easier as we only have to add a reference to the DLL. For unmanaged function though, they mus be made visible to the compiler before being used by either including the file with the declaration or duplicating the declaration (normally not recommanded).