What is the ruling on the Imam reminding worshipers to turn off their phones during the 'Isha prayer? And does this action invalidate the prayer of the Imam alone, or the prayer of all worshipers?

If the Imam speaks intentionally during the prayer, knowing that it is forbidden except for the benefit of the prayer, his prayer becomes invalid by consensus. However, if he speaks out of forgetfulness or ignorance, his prayer is valid according to some scholars, and this is the choice of Sheikh al-Islam Ibn Taymiyyah. The prayer of the congregants is not invalidated by the invalidation of the Imam's prayer if the cause of invalidation is something that might be hidden from them, unless the cause of invalidation is not hidden from them and they continue to follow the Imam thereafter.