I have set one up via an IP Trunk directly without a license which I beleive is for setting up IP extensions. I do not have one setup at the moment but I know I had to play a lot with the configuration. I setup some Ericsson gateways too, which I had to do two trunks, one for each direction since one way worked with fast start but the other did not. The only downside is passing caller ID and some of those types of issues that having two and setting up extensions would probably be better.
Clay