This would be a bad idea as a management tool because it would send a very unpleasant message to the employees. It would send them the message that they are not trusted.
Of course, employees expect that emails will be archived and can become public knowledge. However, they do not expect that their emails will be monitored in real time. They certainly do not expect that their phone conversations will be monitored as well (except in some cases like customer service). Monitoring employees' communications in this way would make them feel distrusted. This would lower their morale and make them less productive.