Apple Watch call failure when only phone is on airplane mode!

This is Working As Designed that the watch uses Bluetooth via the paired iPhone (routing via "cheapest" route) if the link is available, but it doesn't know the iPhone has no cellular service. The real problem is that your airplane mode on the iPhone left Bluetooth enabled, which is a simple settings issue you can correct in about 2 minutes.

The iPhone's system for airplane mode is not exactly intuitive. In a clean setup airplane will totally disable all radios, but you can then go into the phone's setup and re-enable services while nominally remaining in airplane. This state is remembered next time you select airplane mode (crazy? but that's how it works). I suggest on your iPhone with airplane mode ON go into Settings and turn off Cellular, WiFi, Bluetooth. Then disable Airplane, go back to settings and make sure they are all ON again. That should put you back to default behavior for airplane mode with all radios fully disabled when the little airplane appears.

The other place you might want to look is the Watch app, My Watch, General, Airplane mode and set the watch to mirror iPhone = On. That will put both devices to airplane mode when you set it on the either the watch or the phone. However, be aware you have to de-airplane them both individually, because with the radios disabled the first one you set back to normal can't communicate the change to the second device.

Apple’s support document About Bluetooth, Wi-Fi, and cellular on your Apple Watch - Apple Support might help give you a better understanding of how the different communication routes are prioritized.

There is no logical reason (except testing) to run airplane mode as anything except “All Off” and “All On” when you exit airplane mode back to normal usage. As I explained earlier you can set these states in the Settings app (not Control Center) and they will become the default, remembered until you change something deliberately. The default I suggest should make your watch behave exactly as you want, activating cellular when the iPhone goes to airplane mode (unless you also set the watch to airplane). There is a very minor exception if your watch is able to connect or already connected to a WiFi access point which will be used in preference to cellular.

However, be aware the watch’s cellular radio is completely powered down until it loses both Bluetooth and known and connectable WiFi. At that point the cellular radio is powered up and will need time to find its cellular signal and negotiate a connection. That means a call in progress on your watch will be interrupted and the call will drop when contact with the iPhone is lost, until the cellular connection is established.

The information that Branta_uk provided is correct. When you put the iPhone into Airplane Mode, how wi-fi and Bluetooth react is based on the last time you had the device in Airplane mode regarding Bluetooth and wi-fi. I just checked this on my iPhone in both ways. I turned the iPhone into Airplane mode, which immediately put the watch into Airplane mode, however Bluetooth and wi-fi were still on for the iPhone, so I turned them both off. I then turned Airplane mode off, which turned Bluetooth and wi-fi back on again. I turned Airplane mode off again, however this time it turned everything off, including Bluetooth and wi-fi. So, I turned them back on, and turned Airplane mode off. Waited for everything to reconnect, and put the device back into Airplane mode and Bluetooth and wi-fi remained on again. So, it has to do with the end state of the phone when you last used Airplane mode.

I'm not sure what you mean by the statement that wi-fi needs to be turned off when coming out of airplane mode. Are you referring to coming out of airplane mode on the watch? Depending on settings, if you put the phone into Airplane mode, it generally puts the watch into Airplane mode as well. Then each device has to be taken out of Airplane mode individually. The scenario you describe about leaving the phone in Airplane mode and the watch still being connected is if you take the watch out of airplane mode, and the phone is still close by. I agree there are a lot of variables, but having the iPhone in Airplane mode and the watch out, one would think, depending on why you have the devices in different states, why you wouldn't check on the connection status between the two devices. If your devices are close by and connected, then the phone would generally not be in Airplane mode. Although, that is just my opinion, you are free to express your own. You also have the option to voice your concerns and explain the scenario to Apple via the Feedback link, Feedback - Apple Watch - Apple

Are you referring to an Apple Watch that is GPS + Cellular? If so, did you look in Control Center to ensure that you were connected to a cellular signal?

(1) Turning off Bluetooth in the watch control panel does not "really" turn it off. It is still active and will connect to an Apple Watch unless you actually turn off BlueTooth in the Settings App of the iPhone. However, turning on Airplane Mode will actually turn off Cellular, Wifi and BlueTooth on the iPhone. But are you sure that you don't have your devices set that if you turn on Airplane Mode on the iPhone it does not also turn it on on the Watch?

(2) Where does the Watch indicate it is connecting via BlueTooth? There is no indication in either the Control Panel (which does indicate connection via Wifi or a cellular connection) or Settings/BlueTooth (which only indicates other BlueTooth devices like AirPods, Cars, etc.)