If I understand the gw2-9 limitation correctly, the ID you put into GW2 UID is sent as AuthID when you try to place a call, with GW1's UID sent as UserID for all calls on gw2 through gw9. I suppose the secret in getting gw2-9 to work is in finding a service provider who authenticate the call on AuthId and ignore UserID.
Now, in the case of a true termination only account this is the way it should be. It is probably also the mindset Sipura was in when they implemented the function. The logic of this is that if you are using a termination only service it does no good to provide that UserID info on the CallerID screen of the recipient. It makes sense to put gw1's UserID on the CallerID screen of the recipient in order to facilitate a return call.
Unfortunately, the good folks at Sipura did not anticipate that we would want to use this functionality simply for multiple service providers (a la X-Lite/X-Pro). There has been much discussion about this here on Voxilla, but I don't know what it would take to grab Voxilla's attention.
So, for now the workaround (if you can call it that) is to find a service provider who authenticates on AuthID.
