This is the 16th most frequent Lithuanian word.
jų
"Their" (possessive pronoun).
In this sentence, 'jų' is a possessive pronoun indicating ownership ('their').
Jų namai yra labai gražūs.
Their house is very beautiful.
Here, 'jų' functions again as a possessive pronoun, referring to a group as the antecedent whose phone number is under discussion.
Aš neturiu jų telefono numerio.
I don't have their phone number.
In this usage, 'jų' once more acts as the possessive pronoun to indicate that the pencils belong to 'them.'