Same and I think you're onto something, but my childhood predated mobile phones.

I've watched my nephew, who's from a fairly affluent family, call out to his friend who lives next door to come play. They'll chat to each other over the fence and then negotiate with their parents on where they can go play. Neither have mobile phones and I doubt either have used a landline (or have one) so yelling over the fence or knocking on the door seems like the most straightforward approach.

I think phones were the tipping point which changed behaviours - from most people being fairly OK with unannounced or very loosely planned visits (I'll see you Wednesday! == 7:30pm, Wednesday) to requiring a heads up in the form of a call or instant message.