Estimating the increase in traffic is not an exact science, and will depend on factors including organizational size, type of operations, and geographies. It is also important to consider the way in which your staff interact. Do they regularly need to interact with colleagues or others (customers, suppliers, etc.) who they might ordinarily converse with face-to-face? These interactions might need to be replaced with conferencing or phone calls.
How much of your communication will move to other platforms such as websites and instant messaging? Do you have convenient ways of turning these back into phone calls to keep your sales or other processes moving? WebRTC is a powerful technology which can be used to this end.
The starting point is assessing your current traffic, and, depending on the nature of the incident, multiplying it. Major incidents, including those that invoke a directive for all employees to work remotely, could increase your traffic by up to a factor of 1.5.