Yeah this sounds strange to me - every drive thru has had an intercom system like this working outside for decades. Doesn't seem like an insurmountable challenge to me.
1) Sign taped to intercom: "Drive up to window to order". Order at window.
2) Worker standing at the intercom with a notebook and pen and a runner trading sheets between them and the kitchen.
3) "Drive-Thru Closed; Order Inside"
4) Restaurant's phone number posted over intercom.
I'm sure there are a lot more variations I haven't seen. Humans are pretty resourceful when machines fail them but they still want to get paid.