What is the ruling on one who swallowed vomit when it reached the mouth without knowing of the second call to prayer, thinking it was the first?

Abstinence from things that break the fast begins at the break of true dawn. If the second Adhan is called before the break of dawn, then the fast is valid. However, if the aforementioned Adhan is called at the break of dawn, then the fast is invalid, and the day must be made up if it was an obligatory fast, but not if it was a voluntary one. Intentionally swallowing vomit breaks the fast for one who is fasting. Vomit is considered impure by the majority of scholars.