From my own experience:

- You should clearly have a firewall running on the server hosting Asterisk.

- fail2ban should be installed and active

- Port 5038 (Asterisk Manager Interface) should only be open to the server running Standard ERP, if it's not on the same server.

- Port 5060 is the standard SIP client port, and is often attacked. It is recommended to change the SIP client port, however this is not currently possible in Standard ERP. This is registered as a wish and has been approved.

- Put a limit on the number of concurrent outgoing calls, a limit on the countries that can be called, and set a "per day" spending limit. This will prevent a hacker racking up a huge bill.