You're forgetting POTS lines. I can call someone on a POTS line, with a 1992 handset, and it works the same as if I called someone with a brand new GSM phone.
Right. But at the same time, you don't see any of the forklift upgrades that have gone on behind the scenes. All the GSM<->POTS or long-distance POTS calls? They're all being converted to VoIP at some point (private network, but still IP).
