In addition to K Sheehan’s statement I would add that for larger integrations a professional communications individual is a valuable investment. Not only do you need to communicate through a number of channels while ensure the message is consistent you also need to ensure it is understood as intended by your audience. This audience generally includes multiple generations but in international M&As it is also multiple cultural and potentially some work locations that have limited communication vehicles.
Relaying on your local country team is also important; even with a communications professional they will need to actively engage local country representatives to ensure there is an understanding of how messages will be interpreted, how they will be sent and who will support follow-up questions.