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.
In the desktop Bria, try going to Bria's Softphone -> Account Settings -> SIP account -> Topology and then please try all the four different network traversal settings found there. When you try STUN, please enter stun.counterpath.com in the "Server Address" field below and leave the Username and password field blank. You can leave testing TURN if your voip provider didn't provide the TURN server address. You don't need the Server address for NONE and Auto-detect. After each setting change please try to register in order to test that setting.
In the Bria Mobile, go to Settings -> Accounts -> SIP account (disable sip account to make changes) -> Account advanced -> Network traversal strategy -> Strategy. Try both "Server Managed" and "Application Managed". You can also try "Custom Config" by trying the Rport and Outbound option, separately, in "SIP Network Traversal".
Another possible cause is that a restrictive firewall or router can block the request to, or response from the server. Please ensure that Bria 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 provider/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 provider/administrator confirms the server is available and that they are sending the response to you, please include this information in future communications with support.