This is going to be difficult to pull off. Every outbound call from the Iwatsu to a branch location is going to be treated as a trunk-to-pstn call and will follow whatever caller ID info you have set in the stations area. Sending station ID to remote Iwatsu systems is, naturally, SOP. I'd try getting your vendor on-site with a call to tech support to discuss your options. More info, like what other vendors, what SIP providers, etc is going to be needed before you can even start on this.

Sometimes the thoughts in my head get so bored, they go for a stroll through my mouth. This is rarely a good thing.