I am not familiar with the MTA6328Re ATA ...
However registration requires you to register with us.voxalot.com (or eu or au).
If you device has a STUN server field use stun.xten.com there ...
---You may not need to do any of the following if the STUN setting works...---
Look around for a field that report RTP Ports or RTP Range on the ATA and make a note of it
Also behind what type of router is your device sitting?
-You likely want to asisgn your device a static DHCP address on the router based on its MAC address
- You may also want to open and point the approprite ports (5060 + whatever RTP ports your device uses) to the ATA local IP
There is a few setup examples here that may work...
Hardware Configuration Settings - Voxalot FAQ