There's an interesting logic to these alternatives. Just as quantum physics describes the universe alternatively in terms of waves and particles, so linguistics describes language in two ways. Language is at the same time structure and communication. Language as structure faces inward. It is a closed, internally coherent, system.
Phonemes relate only to other phonemes.
But language as communication faces outward. When you phone me, both of us get opened up and changed in the process. Energy and information are transferred between us. Our relationship is different than it was before. Structure and communication are mutually incompatible, just as waves and particles are.