Found one story with other links that might interest you.

I was with a major telecom company and it uses a tool like IM all the time. I dont' know if it used the same port(s) or not or if they kept it internal or allowed external (doubtful). But I also see where the IM of the world have ways around blocked ports...this one isn't the correct link...can't find it today but this mentions part of it - something to do with the http being used (or those ports)