Interconnecting with Ubigate iBG1004 and a BT Sip trunk

OK so I currently have:
1 x Samsung Ubigate iBG1004
2 x Samsung OfficeServ 7200 + samsung extensions
BT Trunk & BT Fibre on BTNet (uk)

I am trying to replace OfficeServ servers with asterisk.
The only piece of documentation i have is an instruction from BT with an IP address to connect to.
I am led to believe Ubigate is a gateway allowing interoperability with various PBXes to be used on a BT Sip trunk.

I know the Internal IP address of Ubigate (which is a SIP gateway I believe)
I know the Internal/External IP of the Officeserv

My plan is to unplug Officeserv, give my asterisk the same IP as the officeserv and plug into the same lan cable (leading to Ubigate).
My issue is registration settings. I have no username/password. I am led to believe this setup doesnt need one.

Do I just set the ubigate to be the sip proxy in my registration settings, with the main SIP server the IP BT has given me?
Any more info would be welcome, regards.