Then, I would suggest to start from the basics: go to Settings > Security > Allowed Country Codes and ensure you are allowed to call those destinations
Then check your outbound rules to make sure there is at least one rule that would match and allow you to make the calls.
Not enough information.- who is the provider ? have you got an outbound rule setup for these types of calls ? does your provider support calls to this country - could they be blocking on the platform.
If you run a Wireshark capture or Verbose logging you can easily tell whether this is a 3CX issue or provider - I normally PCAP Trace, go to Telephony > VoIP calls and see if you are getting 2 call legs or one for this call - if 2 you can see the response coming back from the provider.
If you are getting a Temporarily unavailable message coming from your provider (3CX Activity Log will verify this), then you'd best speak to your provider. Perhaps they are having issues with routing the call.