Why in 1 Corinthians 7:15 Paul uses the perfect tense along the present tense?

1 Corinthians 7:15 New American Standard Bible (NASB) 15 Yet if the unbelieving one leaves, let him leave; the brother or the sister is not under bondage in such cases, but God has called [a]us [b]to peace. Footnotes: a. One early ms read…