With wifi and mobile data off, iPhone still received and sent Whatsapp messages

A very strange thing happened this morning, my mobile data was off and Wifi was unconnected while travelling on a train. But I received multiple messages via WhatsApp. I was also able to send messages that were received but no images were sent (I tried to send a screenshot of connections being off but receiving messages). I thought it had randomly connected to a network but the Wifi said not connected in settings. After switching off Wifi (hence Wifi, Bluetooth and mobile data were all 'Off' in settings) I still was able to receive and send messages. The only connection was cellular, but no 4G/3G or Wifi. The moment I turned on airplane mode this stopped, once I turned it back off I could no longer send or receive messages until I connected to a network again. I have found this very strange, and cannot find in the Analytics any data relating to connections to networks made at this time (is this possible to see what network I was connected to at a specific time?). The only logs in Analytics Data at this time was SiriSearchFeedback at the exact time when I started receiving messages without being connected to any network. Does anyone have a logical explanation for data transfers without explicit connections? Model: iPhone SE (2nd gen)\ iOS: 17.4.1