This is something I think about with my kids when they get to that age. I was calling my friends (on their landlines, using our landline) regularly by then, talking to their parents en route to getting them on the phone, and arranging visits. My kids won't grow up in a world where that's something that happens, and I'm not sure how to support their social independence in a world where (as you say) it seems nigh-on-negligent for them to have their own phones.

There is a nascent movement of families bringing back landlines for exactly this reason