SIP VoIP Trunking on BCM

SIP is only supported on BCM Succession 3.5. If you have a 3.5 Box then the info you are seeking are in the docs.
ghostrider42us

I'm not sure I understand the docs, but I think the docs say that a BCM can only talk to another BCM using SIP. Is that correct? There appears to be no way to get it to register to a SIP proxy.

Can it register to a gatekeeper using H.323?

The BCM talks to other BCM's via IP Trunk keycodes.
You may choose the signalling methods either H.323 or SIP.
Standard trunks supporting MCDN are H.323
SIP is less functional due to early release of code.
BCM supports 3 methods of H.323 resolution:
Direct
Gatekeeper Resolved
Gatekeeper Routed

Only supported gatekeeper that comes to mind right now is the RadVision H.323 stack.

Refer to Using Radvision ECS 3.2 GK as the gatekeeper and Using CSE 1000 as a gatekeeper for specific information about configuring the gatekeeper for H.323 trunks.
Network note: If your private network contains a Meridian 1-IPT, you cannot use Radvision for a gatekeeper.

SIP trunks communicate between Business Communications Managers. The addressing for the remote destination is described in Setting up SIP trunk subdomain names.