Most servers / setups will require you to be registered to be able to receive incoming calls.
Can you try to go to your account settings in Zoiper and click on register.
Then immediately make an incoming call, does it work for this call ?
If this call did arrive, try changing the transport to TCP instead of UDP. (This will only work if your provider supports TCP, some do not).
If TCP is not an option, try reducing the reregistration time to 30s.
If this call also did not arrive, try changing the settings for RPORT and/or STUN in Zoiper and see if it works now.
If both suggestions do not work, contact your provider and check if everything is ok on their end.