Twenty-first-century teaching and learning provide online resources accessible to foster learning experience across time and spaces in an informal context. Hence, enhancing learners' voluntary adoption of digital devices is crucial to augment the promising assistance of technology for English language learning. This study sheds some light on English for Specific Purposes (ESP) learners' voluntary digital devices for out-of-class language learning. A total of 267 Indonesian ESP learners participated in this study through a survey and semi-structured interview to examine how they were involved in informal digital learning of English beyond the classroom. The results showed that the learners used digital devices for both intentional learning to strengthen coursework and incidental learning to expand access to the target language. It was also found that the learners frequently employed digital devices for personalized aspects of the language rather than authenticity and connectivity. As for the digital device selection, the learners mostly preferred mobile phones to computers and tablets to carry out out-of-class English language activities. These results provide fruitful insights for educators on supporting ESP learners to engage in informal digital learning of English and achieve possible synchronicity between teachers' designed digital activities and learners' actual learning practices.