Is this a very specific ring tone that is only used for a phone call or could it be a standard sound that is used for other stuff as well?
To expand on @Volker: Maybe an app that sets some sort of alarm? Did you check if any alarm settings are active? There seem to be (old) apps that use the alarm mechanism as a work-around to wake themselves up, then move the alarm to the future. If such an app would get triggered by the alarm, acknowledge it pretty much immediately and move it to the future, you might get the effect that you describe.
A similar problem was described here:
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