There are a few risks in using this solution. Mainly, you re opening up some of your communication to the external world (this traffic is going over the public Internet), but that s really not a significantly greater risk than talking on the phone (particularly if the phone traffic is going outside of your private company switch). You re also taking on some risk that the tool will be abused by employees (for instance, talking to a boyfriend/girlfriend/spouse at some other location), but if that s a real risk for you, the root problem probably exists whether you re using instant messaging or not. Here are a few of the most popular.
More on unified messaging and unified messaging application