Hello, the SIP 408 means that the register request cannot reach the VoIP server or the response cannot reach you. Try altering STUN and rport configuration in "Config"-"Accounts"-your account-"Network settings". Save changes and try again. Try different settings with STUN and rport and make sure you are using the correct transport type(TCP,UDP or TLS).
You must make sure that the default ports are not blocked/filtered/rewritten by a firewall/router device. Default ports are:
SIP port is 5060
IAX port is 4569 UDP
RTP port is 8000 and above UDP
Default STUN vallues:
Server hostname /IP :stun.zoiper.com
Port :3478 UDP / TCP
Refresh period : 30