[Serusers] SIP to PABX

Hello, I'm looking for some sort of gateway which can REGISTER itself at my SER server, can respond to 407 messages and is directly connected to a pabx (either using ISDN-2 or E1). So basically a SIP <--> ISDN-2 / E1 gateway which acts as a user agent (or multiple). The reason for this is that we don't want to replace the current pabx (which only has ISDN-2 or E1 for external communication). I've found some devices from Mediamatrix (http://www.mediamatrix.com), but I would like to know if any more of you have come across the same problem and which devices you use. Some wonderfull ASCII art: SIP |----| ISND-2 or E1 |----| -------| |-----------------| |-- internal phone network |----| |----| Gateway? PABX The reason I want the gateway to be able to authenticate itself is that the gateway is located at another location(so the SIP in the picture above will be going onto the internet), so I don't want people to be able to spoof the IP of the gateway. Kind Regards, E. Versaevel