First of all, the patent system should be restrained as much as possible, and the technical scheme should not be described by function too much. Because functions are realized through specific structures, if you are given a functional patent protection, it means that all structures that can realize this function belong to your protection scope, which limits the innovation of others, because what we invent is nothing more than something useful to us, that is, function. Because there are many structures that can realize the function, you only give one or several specific structures to realize this function, and it is most appropriate to limit your protection scope with your specific structure. You think that mobile phones are nothing more than those functions, and all innovations are within your boundaries. How can you be given such a right?
I think you misunderstood what you said about protecting some functions as patents. It may give a technical solution to realize a function, and its goal is to make the mobile phone have this function. Its protection scope does not mean that the mobile phone has this function or its scope, but the specific technical solution to realize this function is its protection scope.
In addition, the patent can't absolutely require you to write down your protection scope in terms of structure, because if you simply describe the structure, some structures may be easily replaced by other structures, such as one part sliding on another part along the guide rail. If it is only written in the form of contact sliding, your technical scheme may be ok to slide with a roller. At this time, it can be said that one part is slidably connected to the other along the guide rail, which is "slidable".
The general technical scheme can't be purely functional description, but some of its structures can have functional description, provided that the structural description can't be used, or it is better to use structural description than functional description. For example, the functions of some of them, you have given many alternative structural schemes, which are difficult to unify in structure, so you can consider using functions to describe them. The scope of protection after authorization of the function description includes all technical solutions to realize the function, that is, to protect other structural solutions that you did not expect.