Hello,
Error 404 is returned when the number cannot be found or is incorrect, while 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 STUN Settings -> Accounts -> Your registered account -> Network Settings.
If you use WiFi, please make sure that non of the ports used by Zoiper are blocked from your WiFi 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.