If your account cannot register for some reason, Zoiper will display an error message below the account name in the Accounts section. Please check for that error message.
Try to alter "STUN" and rport for your account and try to register again. Both options are located in "Configuration"->"Accounts"->your account->"Network 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 values are:
Server hostname/IP: stun.zoiper.com
Port: 3478 UDP/TCP
Refresh period: 30
Please note, that some Mobile providers restrict the use of VoIP into their networks.