Hello, SIP 408 is generated when the register request cannot reach the VoIP server or the response cannot reach you. This is in most cases NAT related issue and disabling/enabling STUN and rport could help. Both options are located in "Settings"->"Accounts"->your accounts->"Network Settings". Try different combinations with both settings.
If using Zoiper with WiFi, please make sure that the routing device is not blocking ports used by Zoiper. The default ports used by Zoiper 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
Do not hesitate to contact us again if you would need any additional information.