First telnet IP with port, if that does not work then test your firewall settings for IP blockage. If not, then get tracepath to your server from your client.
To do Telnet from admin panel, go to Sidebar -> System -> Site Tools
Click on Telnet Tab. Enter IP/Host with Port. Click on Telnet.
If you want to tracepath to your client’s server then click on Tracepath Tab. Enter IP and click on Tracepath button to get results and share it with your client.
If client is not able to connect to your server then ask them to do the same as mentioned above.