The requirements for dialing SIP from outside your network to an internal device are pretty straight forward. In your case you have a VBP. When the call is received by the VBP it will forward the INVITE to the SIP registrar configured in the VBP for username/IP address resolution.
I tried calling the address in your post. What I found is that there is a problem with the Session Expiration timer. When I call I use a SE of 1800. The call is being rejected by a device calling itself TANDBERG/4132 (X8.7.3) with a SIP 422 - Session Interval Too Small error
SIP/2.0 422 Session Interval Too Small
Via: SIP/2.0/TCP 18.104.22.168:5060;branch=z9hG4bK-3836-d84654616413ea008f103a104e8ad034;received=22.214.171.124;ingress-zone=DefaultZone
CSeq: 1 INVITE
From: "Michael Bromley" <sip:email@example.com>;tag=EDD13B86-23316633
Server: TANDBERG/4132 (X8.7.3)
Warning: 399 126.96.36.199:5060 "Session Interval Too Small"
The Tandberg VCS is declaring a minimum Session Expiration timer of 2000 where I am using an SE of 1800. 1800 is the usual default for Session Expiration so if you can find where to reconfigure that on the VCS to 1800 I believe your SIP calls may start working.
Michael K. Bromley, CVE, CCENT, WCNA, VCA-DCV Technical Lead Infrastructure Technical Support Center