You misunderstand. Your prepaid carrier does not provide international service, hence the need to purchase additional SIM cards.

for those who want to keep their # for receiving calls overseas, prepaid is not an option absent workaround i.e. google voice, and additional SIM cards. only postpaid carriers offer international roaming because the invoices can be substantial. A prepaid carrier would not know how much to charge you for your roaming when you could talk 5 or 500 minutes.