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You don't need to "buy" World Traveler but you MUST have International Roaming
You're right: since the Inspire 4G includes the GSM 1800 band, then it can function for voice (phone) in Costa Rico. BUT......................
You won't be able to use it unless you have requested AT&T to allow International Roaming on your phone account.
You don't HAVE to "buy" AT&T Word Traveler; that $3.99/month fee simply means you pay less for voice minutes than you would if you didn't have it.
Per the AT&T website: their rate is $2.89 per voice minute without World Traveler and $1.99 per minute if you pay the $3.99/month for World Traveler.
(Obviously, the phone won't work with a local Costa Rica SIM card unless it's unlocked. Since you mentioned your phone is not rooted and since AT&T won't unlock Inspire 4G's until sometime in August, then I assume it's not unlocked unless you purchased an unlock code from an interent site.)