Hello and welcome to FF Kate! Your frog can be calling for a mate or just being a frog and calling to interact with environment. You can get him company; but they must be a similar size to prevent cannibalism and quarantined for at least 30 days to ensure your established frog does not catch anything from them. Also, it's important the enclosure is big enough for all the frogs since they (specially females) can grow quite large. There is more information on them in here: Frog Forum - White's Tree Frog Care - Litoria caerulea. Good luck !