More info: DTMF does not work over WiFi, but does seem to work (at least sometimes) over cellular. If I turn off WiFi, I can use the DTMF tones in a call. I'm with a plan that switches automatically between WiFi (if enabled) and several cellular services, depending on which signal is the strongest. I have read other reports that the reliability varies even between cellular services. I'm not sure which cellular service it chooses here.
