This issue appears mostly because the session was not actually established and is in most cases network(NAT) related. For example when you receive a call, Zoiper should reply with an acknowledgement which probably never reached the server for some reason (or the ACK from the other side never reached Zoiper).
This in most cases is because some routing devices / firewalls have built-in "SIP ALG"(Application-level gateway) functionality which is implemented to resolve NAT(Network Address Translation) issues, however the implementations are wrong and break SIP.
If you need additional, more technical information you can find it on the link below at the "SIP ALG problems" section:
You need to first disable SIP ALG under your router configuration, if you do not know how to do it, you will need to contact your Network service provider / administrator for assistance.
You should also make sure that ports used by Zoiper are not being blocked or filtered by your firewall / router and are properly forwarded:
SIP port is 5060
IAX port is 4569 UDP
RTP port is between 32000 and 65535 UDP
Default STUN values:
Server hostname /IP :stun.zoiper.com
Port :3478 UDP / TCP
Refresh period : 30
If you do not know how to configure your routing device / firewall, please ask for assistance your network provider / system administrator.
Altering STUN and rport / rport media options in different combinations could also help.
You may need to disable or enable STUN and rport in different combinations according to their current state e.g. if they are enabled try disabling them and vice-versa.
You can find the per-account STUN / rport in Zoiper -> Settings -> Accounts-> your account -> advanced sub-tab. \
You can also disable "Open random port abover 3200" for SIP, under Zoiper -> Settings -> Advanced -> Network sub-tab.
Additionally you can try using TCP or TLS transport type instead of UDP. (If supported by your VoIP provider) You can change this by going to Zoiper -> Settings->Accouts -> Advanced -> Transport - choose the necessary option.
Note that if your VoIP provider/ PBX does not support different transport types than UDP, you will be unable to register your account and you will need to revert the setting.