Think I may have been missing the obvious, having not used ISDN before on this system.

I am using the help section of the PC Pro programming tool, and it made it clear that the DDI would follow the lines first AA message from the VRS.

There is then a small text line at the end saying from version 6 onwards, you can direct the DDI to a VRS message using 500 - 505 being VRS message 505 for example.

At least this is the way I read it!