If you are physically in an area with no service, then no it will not work. It cannot work if there are no cellular towers (or wifi if the device is setup for WiFi calling) as it cannot place a call at all then.

Does emergency SOS work without service?

When you start an Emergency SOS call while abroad, your watch connects to local emergency services, but it doesn't send your location or a text message to your emergency contacts. In some countries and regions, international emergency calling works even if you haven't set up cellular service on the watch.

Can you call SOS with no signal?

Yes, you can make emergency phone calls without a network signal. When your phone loses signal from your provider's network, it automatically connects to the strongest network that it can find in order to allow for emergency calls to go through. However, since it is not your network, you cannot make other calls.