Hey @tiziano, at the moment it is not possible to do this with Rasa Open Source.
The main problem here is that Rasa Open Source has dependencies such as TensorFlow 2.0, which you would need get working on your Android phone somehow. As far as we know, this is currently not possible, or would take a very large amount of work to do. There may be other dependencies as well which are distributed as binary packages (wheels), which you might not get working on Android as well. They also tend to depend on the CPU architecture (x86-64, ARM, etc.), which makes it even trickier.