Maran the Shulchan Aruch (Orach Chayim 96:1) ruled that it is forbidden to hold anything in one’s hand during prayer—neither Tefillin, nor a book, nor a knife. This is because a person fears they might drop the object, causing them to focus on the item and distract their mind from the prayer.

Maran the Mishna Berurah (ibid. 5) wrote that this does not apply only to those specific items, but to anything that disturbs one’s concentration during prayer. Therefore, it is forbidden to hold anything in one’s hand, especially a cellphone, which is the greatest possible distraction to a person’s focus, as it could vibrate at any moment.

There is even room to say that a person holding a phone is considered merely “occupied” (Mit’asek) with the prayer rather than actually praying with intent (Kavana). According to the view of the Grach (Rabbi Chaim) of Brisk, such a person would be required to repeat their prayer. Even if we hold that one does not need to repeat the prayer, Maran the Chida (Machzik Beracha 1) brought in the name of Mahara Galanti that such a prayer “is not heard in Heaven,” may Hashem (God) have mercy.