apcyberax gave a good reply. virgin bt or whoever negotiate call costs with different countries. if country A is charging an acceptable amount then virgin/bt etc will go with them and include in the free minutes. if country B charge more then it isn't cost efficient nor does it make business sense to include it as part of free minutes.
i spent five minutes looking and couldn't find a provider that has free calls to malta as part of a free minutes package. maybe someone can confirm otherwise.