A 408 error means that Bria is sending a registration signal to the domain you've configured, but never receiving a response. This usually happens for one of three reasons:
1. The signal is being sent to the wrong server
2. The server is receiving the signal, but is unable to process it
3. The server is processing the registration, but the response back to Bria is being routed incorrectly.
For cases 1 or 2, I recommend you contact your VOIP service provider and confirm your SIP account information, and that their server is receiving and processing it.
However, the third case is generally the most common, and you can usually work around it by changing your Firewall Traversal Method in the Bria admin portal. Log in to the admin portal, go to the Voice & Video tab, and edit your server configuration. In the "Service Settings" tab, try changing your Firewall Method to one of the available options. If you set it to STUN, you can use stun.counterpath.com as the firewall server URL (it does not require a username or password). Remember that once you make a change in the admin portal, you need to log out and back in on the Bria app to download the updated settings.