Obtain a US SIM card for Cell Phone
Plan to bring your unlocked phone to the U.S. and obtain a SIM card upon arrival.
The ISO offers a temporary SIM card at their office, so stop by to pick one up. You can decide to keep this plan temporarily or more long-term.
UPDATE: While the ISO is virtual, you can fill out this FORM and request a SIM card to be mailed to you free of charge from CampusSIMS (including to your home country, 2-5 weeks before you arrive to MIT).