Error 408 is received when Zoiper can not connect to the VoIP server. There are two possibilities - the server can not receive the sent packages or Zoiper can not receive the answer from the server.
You can try to alter the STUN settings from Settings/Config -> Accounts -> Your registered account -> Network Settings.
Please also make sure that non of the ports used by Zoiper are blocked from your routing device or Firewall. The ports used by Zoiper are as follows:
SIP port is 5060
IAX port is 4569 UDP
RTP port is 8000 and above UDP
Default STUN values are:
Server hostname/IP: stun.zoiper.com
Port: 3478 UDP/TCP
Refresh period: 30
If you using mobile internet connection, please contact your mobile connection provider.