Thanks for this tip, it works perfectly.
This is also a good tip for de wiki site I think
http://wiki.pbxnsip.com/index.php/Microsoft_Exchange
Section: Configuring the pbxnsip Server for Exchange.
1. Create a new trunk to connect to Exchange: Using the pbxnsip Domain Administrator, select the Trunks tab. Name the new trunk "exchange gateway" and set its type to SIP Gateway. Click the create button. Click the edit icon next to the Exchange Gateway. In the Domain box, enter the FQDN or IP Address of your Exchange Server (i.e., exchange.company.com). In the outbound proxy box, enter: sip:exchange.company.com:5060;transport=tcp but replace exchange.company.com with the FQDN or IP Address of your Exchange Server and set Remote Party/Privacy Indication to RFC3325 (P-Asserted-identity).
The example below assumes the address is 100.200.100.200. Click the "Accept redirect" radio button and then click save.
Screenshot needs an update