Not sure how but for a brief moment earlier, when doing a tracert to the Bandwidth SIP Proxy, it looked to be going out the right path -- tested a TW call and had two way audio (TW from one of the corporate offices, which have a VPN tunnel between them, but still hooks up via the MBG public IP).
Went back to the shell a moment or two later, did a tracert, this time back out eth1 (WAN) interface. No more two way audio.
I also called Bandwidth and had them also add the public address for the MBG (instead of just the PABX), in case that would help.