The error 408 Request Timeout indicates that the client is not receiving any response from the server to which you are trying to connect. This can happen if your request does not reach them, or if they do not respond to you/their response does not reach you.
One cause for this error may be incorrect server information (domain, server address) entered in your SIP account. Double-check this information and try to connect again. You can also try adjusting your topology settings, as these may prevent the response from a server from reaching you.
Another possible cause is that a restrictive firewall or router can block the request to, or response from the server. Please ensure that VoIP is a permitted application through all firewalls and routing devices in your network and operating system.
If the above techniques have been tried, you may wish to contact your VOIP service provider or PBX administrator to see if your request is reaching them and if they are having a temporary outage of their service.
If you have tried all the strategies above, and your VoIP service provider or administrator confirms the server is available and that they are sending the response to you, please include this information in future communications with support.