Hello,
If port 22 is not blocked in any way, the router at the remote site is configured to forward to your Asterisk server, you have SSH installed on your remote server and you know the WAN IP to connect to, then you can connect to your remote server with SSH (preferred over Telnet, since Telnet is pretty insecure) with:
Needless to say you'll also need a user on the remote system with the necessary permissions.
Kind regards,
Eric