When you have both the legacy Function object and the newer function blocks in the same project, it's not clear which kind of function should be invoked by c3_callFunction
(note c2_callFunction is just another name for the same function). IIRC, the legacy Function object "wins" and gets invoked, for backwards compatibility purposes. As there's no way for that function to specify which type of function to invoke, it's not possible to specify that it should invoke a function block instead.
The legacy Function object is deprecated and will no longer be supported, so your best option is to remove it and replace it with built-in functions.