Make sure you comply with minimum requirements, that your telephone or software are correctly configured and that your Internet connection is working properly.
If you are using a firewall, configure it as follows and try again:
- UDP & TCP through 212.97.59.64/27 and 193.227.104.0/24 on the ports 5060 and 5061 with the relative answers;
- UDP through 212.97.59.64/27 and 193.227.104.0/24 on the ports from 10000 through 65000 with the relative answers.