In your sentence, 'there' is used as a pronoun which roughly means 'there exists'. We want to use the noun 'there' (corresponding to वहाँ) which means 'that place'. So, switching around the words, "Is anybody there" seems like a good enough translation.
Hindi does not differentiate much between 'someone' and 'anyone'. You can use "कोई" and "कोई भी" respectively for the two but in sentences like the one we have, both 'someone' and 'anyone' work.