However, everything goes to one device, in this case a hard phone. What if I add more devices? At the moment I have Asterisk on a virtual server with digital ocean.
If you have devices behind NAT connecting to a public Asterisk server then it is unlikely you need to set up any port forwards at all, if the remote Asterisk is configured to expect the devices to be behind NAT. It can keep the NAT mapping open so everything works as expected. It’s only in cases where the router is very broken that it can cause a problem.
The qualify option is what keeps the NAT mapping open for signaling. The above ensures that we ignore the address information they provide in their SIP messages.