I turned Airplane Mode on, yet someone was still able to call me. How was this possible when Airplane Mode supposedly shuts off all airwaves?
That is VoWi-Fi. VoWi-Fi doesn't necessarily require your cellular radio to be "ON" in order to make calls. The SIM card being "present" in the device is enough
The content below is from a comment I wrote on the same question as a reply
The SIM card serves as an "identification module" which enables your "smartphone" to process phone calls, either via cellular radio, or via VoWIFI. Your SIM card being present and working (working as in it sort of informing the phone that you "are" subscribed to a plan) indicates your phone can make phone calls using your Telecom provider's exclusive network services, the device depending on the circumstances/configuration decides how to proceed with the call- either via VoLTE or HSPA or Edge (these three uses cellular radio) or via WiFi (VoWIFI aka WiFi calling)
Which means nothing is "bypassed" here. Your phone can't make/receive WiFi calls while in airplane mode if your SIM card isn't present or if your SIM card is present but you don't have any active subscriptions