I'm not sure that's anything new. Sure, I'll call a restaurant if there's no easy way to book online. But this doesn't work as well when I'm traveling overseas (language, cost of phone call) and, as you say, phone lines are busy, get put on hold, phone not picked up, etc.
Also, with an app, I can see upfront what my options are and potentially see if I have better choices elsewhere.
Unless you call wgile tgey are slammed during lunch and dinner rush, this is absolutely not true.