Ironically, a watch is just fine for taking phone calls. The Apple Watch can do it; I've used it a couple times when my phone was in the other room or I was driving and it was a lifesaver. It's actually a fair bit more convenient than speaking into a phone since it's lighter. Only problem is it's always on speaker, but in many cases where you don't have a phone handy, that's what you'd use anyway.

The reason you still need a phone is because we now use a phone for lots of tasks that used to require a desktop computer.