As you might now relationship field are not available inside trigger so you need to query that information in your trigger and from there you can match the information in trigger.new loop. Following code will demonstrate it
//get all the account from current opporutniy
set<id> setAccountIds = new set<id>();
//Get all the account id
for(Opportunity opp : Trigger.new)
{
if(opp.Accountid!=null)
setAccountIds.add(opp.Accountid)
}
//query the account object for details and add that into map
Map<id,Account> mapAccounts = new Map<id,Account>([select id,Billicountry from Account where id in : setAccountIds])
//loop over opporutnity and check for account details and make call out
for(Opportunity opp : Trigger.new)
{
//Check for the logic
if(opp.Accountid!=null && mapAccounts.containskey(opp.accountid) && mapAccounts.get(opp.AccountId).Billingcountry)
{
//Write your call out logic here
}
}