asterisk users - Apr 2005 - How do I add an IP to an Exten

This works from-pstn just fine.
exten => 10 digit Inbound PhoneNum from the 
pstn,1,Dial(sip/Phonenum@Proxie IP address,,r,)

How can I add the Variable exten with the proxie IP address. I want the 
exten to call my proxie.
exten => _.,1,Dial(sip/${EXTEN}@Proxie IP address,,r,)