As you may already now Exchange 2007 has UM functionalities. Besides the commercial packages there are several free products, one of them is Asterisk. In the previous version you needed SIPX between Exchange and Asterisk to get it working. In the new version Asterisk 1.6 this is not needed anymore. Geoff Clark’s has created a tutorial about this, if you interested then have a look at his site.



Johan Veldhuis