Wi-fi "working" could mean lots of different things. For example, maybe
the phones can connect via wi-fi to the modem but the users haven't yet
tested that they can actually get an Internet connection (by using a web
browser).

One user absent-mindedly logged into her office remotely and was
connected. That's how they found out they had wireless Internet.
That's her work phone and it doesn't have cellular service.

VOIP service is separate from Internet service. That's why when you
call to report an outage they ask if the other service still works (to
know the physical connections and cabling are still okay).

Unclear if the work phone is a landline phone or a cell phone but with
no cellular service (no voice or data service plan).

It's the latter.

One of my troubleshooting tips was to confirm the expected SSID of the
wireless connection.

While you could dig into the modem's configuration to see, for example,
if the wifi connection was subnetted away from the Internet access (it's
very possible to use subnets within the intranet that grant no WAN-side
access, and the same for the Ethernet wired ports), it would probably be
much easier to reset the modem to its defaults.

Thanks VanguardLH. As always, your reply is thorough and
well-researched. However, at this point we're not trying to fix
anything. Just waiting on AT&T.

I'm just baffled as to how all the smartphones have WiFi even though
the router's Broadband light is flashing red. The Ethernet-connected
desktop is the only one without Internet.