Buy a virtual number from your cloud telephony service provider.
There are 2 methods for authentication: IP whitelisting, or auth credentials.
Currently SubVerse SIP server does not have a static IP, so if you opt for the IP route, you need to whitelist all the IP addresses in the range like this: 0.0.0.0/1, 128.0.0.0/1.
It is recommended that you also setup Auth Username and Password for secure communication.