SIP trunking with Switchvox?

Yes there is, you can peer two Switchvox machines together.  This will allow extensions from one system to use another system's provider for outgoing calls.  The following is a link to our Switchvox Manual which will walk you through setting up SIP peering (page 176, Switchvox SIP Peers):

http://www.switchvox.com/sv?page=customer_center