When he called his Lord secretly.
He called his Lord, a secret call.
when he called upon his Lord in secret
When he called upon his Lord, crying in secret
When he called/cried (to) his Lord a hidden/secret call/cry
When he called his Lord secretly in a low voice
When he softly prayed to his Lord
When he called upon his Lord in a low voice
When he called upon his Lord in a low voice (charged with politeness and submissiveness)
When he called upon his God, crying aloud (in humble supplication)
When he called out his Lord (Allah) a call in secret