How can I make my app to fetch (transactional) SMS when it is neither running in background nor foreground?

I have an app which fetches (reads) SMSes when it is running in foreground or background. However, it does not fetch SMSes when it is killed/ stopped. How can I make it to read SMSes even when the app is closed/ not in use. (The way Truecaller reads SMSes even when it is not running)

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