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.

Unless they also disable their cellular data connection, they
won't know if the Internet traffic they generate is over the wi-fi
connection to the modem or through a cellular data connection.

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