This is the first paper that provides correlational evidence about how interacting with an online allophonic transcription tool helps learners of English as a Second Language (ESL) to improve their phonological awareness. The study investigates 55 advanced ESL learners at a Polish university enrolled in a course in English phonetics and phonology. The course placed heavy emphasis on reading and writing allophonic transcription based on the International Phonetic Alphabet. Apart from obligatory practice with traditional pen and paper worksheets, learners could also practise with a supplementary custom-designed web application that allowed them to enter the phonemic transcription of any word in order to receive its allophonic transcription. The results show that using this tool at least once during the course is associated with an expected increase in midterm test score of 5.03 percentage points, 95% CI [-10.61, 20.67]. The estimated benefit is higher for learners who space their usage of the tool; each additional distinct day of usage is associated with an additional increase in the expected midterm test score of 2.62 percentage points, 95% CI [-3.25, 8.49]. Additionally, some learners practised transcription on non-words, and these learners were observed to perform better on phonotactics-related assessment.