Hi Stoney64,
I believe that the IP Trunking that Siemens are currently using is bespoke to Siemens, so will only talk DX to DX, but I believe that a new version is on it's way that talks SIP. So you should be able to connect any compatible SIP product to it.
Historically a DX-R has been utilized to connect to the IP-MPLS to site, with a traditional DLI / DPNSS connection to other PABX's.
Hope this helps,
CRM026