A standalone cloud PBX solution like VPBX is solid, but it’s not a one-size-fits-all solution.
To scale effectively and handle NAT issues, you’ll likely require a proxy/SBC. It doesn’t have to be overly complex; a Kamailio instance facing the internet in front of the PBX can significantly improve performance in many situations. In some instances, a local device within the customer’s network may be necessary to navigate CGNAT and tight firewalls.
It’s essential to recognize that there isn’t a single solution that fits every scenario.