SOS message sending “Failed”
When I used the SOS notification for an emergency. I received “message sending failed” notification for all my contacts assigned in Health.
iPhone XS
I still believe it is low power mode. Do you have an SMS/MMS plan for your iPhone? I've never used the SOS, and I've not seen how it sends the messages. This support document, Use Emergency SOS on your iPhone - Apple Support just states it will send a text message, and since emergency services uses cellular, it may not send an iMessage, but a text message (SMS) which requires you to have the SMS plan on your phone. Check with your carrier to see if they can explain why the messages did not send.