The Draytek DV2750 router that we recommended at the end of the process to the customer was the one that we stuck with, as they have a reputation for being significantly more reliable than the two that the customer tried before. Unfortunately the usual "How to get online with VDSL2 on the Chorus Network" instructions that come with the Draytek DV2750's was not apparently included in this one. As such, the 3rd step of "Disable hardware NAT" was therefore missed. This was the culprit of the issues.
Our customer had even contacted SnapperNet Support (They're usually fantastic!) who hadn't picked up on that either, as well as getting in a 3rd party IT Support company to investigate. In the end, we sent out a technician who promptly found the issue and resolved the situation. Since then there have been no issues, it has worked reliably handling multiple concurrent calls for over a month now. To our knowledge there has been no call dropping and no outages since.
We work with a all Telcos in New Zealand, not just one or two and typically leave the network side to either the Telco or the customer's own IT resources. Hence, we don't typically recommend, supply or configure routing equipment. In this instance we did recommend the router and ended up assisting with configuration. Perhaps, in retrospect, we should have done so at the beginning of the process rather than at the end of it.
on behalf of the Vadacom Team.