Usually, if calls to most of the destinations are ok and certain destinations only can not be dialed, the issue is to be found on the server.
Depending on the server configuration local calls can be allowed and international calls can be stopped. This does not depend on Zoiper as Zoiper is only sending the number it wants to reach to the server.
The problem could be with Zoiper only in case the number was somehow altered by Zoiper during dialing.
Please contact our support team by email, they will send you instructions about how to prepare a packet capture and a log file.
When we have them, we will be able to assist you in this investigation.