I have to look at this. I have 2 SIP peers from the telco, two 3300 IPBX for redundancy, but only one MBG.
Calls come on the first SIP peer, if I shutter the first IPBX, calls get to the resilient phones. If I shutter the first SIP peer, it works too. However, I can't emit calls from the second IPBX to either SIP peers, only incoming works.
If only the client wasn't so cheap and had forked for another MBG !