You can set the default content that will be applied to all communication Emails and SMS.
Navigate to the Account Setting page from the Admin menu
Add the SMS content to the Initial SMS Content
Add the email content to the Initial Email Content
Tick Save
To note:
All users still have the ability to override the Email and SMS content at the campaign level. When the administrator (publisher) constructs the campaign they can override these settings in the Delivery step of campaign creation.