except
The German word 'außer' is used in the same way as the English word 'except'. It is a preposition that introduces something that is not included in a general statement. For example, 'Ich esse alles außer Käse' (I eat everything except cheese).
Nobody has called except you
This sentence indicates that no one other than 'you' has made a call, hence using 'außer' to express the exception.
He works every day except Sunday
Here, 'außer' is used to exclude 'Sunday' from the collection of 'every day', meaning the person does not work on Sundays.
Except when it rains, I go running
In this case, 'außer' introduces a condition (when it rains) that stops the speaker from going running. When that condition is not met, the speaker goes running.