I have no problems making calls using the google voice app.
I open up the GV app, select "Dailer", enter the # or use the address book, then select "call".
I admit I do not call out of the USA with my phone, but what Google Voice does is call a number within the states, and then that will route your call to the right person. The routing will also allow the other party to see your GV # (if you have it programmed that way).
The only reason AT&T would have charged you is if the GV routing # was one outside of your included calling area. Perhaps calling Jamaica, GV requires the call to be routed to a call center outside the USA.
That being said, you can always just open up your native iPhone "Phone" app to see the most recently dialed #; or check your phone bill to see what # GV is actually dialing when you made those calls to Jamaica.
Hope this helps.