While United Mobile is a good value in some countries...roaming cost is prohibitive other than maybe for a quick couple of "I didn't get lost in the Bermuda Triangle" calls before you buy a local prepaid sim or calling card.

Only good deal on United Mobile in Bermuda would be SMS text service because that's a flat rate. My Iridium satphone works in Bermuda for less a minute than a United Mobile call...and that's not saying much.
Would suggest either:
1) Buy a Digicel Flex prepaid sim card and some airtime vouchers. Excellent coverage on the island. Used to be Cingular Bermuda but was sold with many non-US Caribbean Cingular properties to Digicel of Jamaica. Good thing is most Digicel prepaid cards allow roaming in other Digicel countries (in the Caribbean and Central America) for a reasonable rate, and even on Cingular USA for a somewhat higher rate. Think Bermuda to US direct dial on Digicel Bermuda used to be 75 cents a minute and local calls were a quarter a minute, but may be less now. Could use with a Bermuda calling card or non-Bermuda callback service to reduce international call cost. Digicel sim cards can be recharged from other countries using their webflex web refill system (either directly or through a dealer).
2) Low tech but reliable solution is buy a prepaid international calling card from a market or a vending machine in a store or at the airport. Usually around a quarter a minute back to USA from a payphone; think some are less. Ask if using from a hotel roomphone what the hotel surcharge is, if any. Think a better solution than non-Bermuda phone cards, because some are blocked or require dialing a toll number for access.
3) Possible to use Skype-Out via Wifi or ethernet connection if you can get a free or low cost connection (I know some 802.11b wifi service is available on the island). About 2 cents a minute back to USA on top of the internet connection cost.
Also a non-comm suggestion: buy a combo bus/ferry pass and schedule brochures at the table by the baggage claim area. Even if you take a taxi to/from your hotel(they discourage big luggage on the bus because they don't have much storage room; single small carry-on would likely be OK), using the bus/ferry service during your stay can get you just about anywhere on Bermuda easily and cheaply (plus the ferry service is a lot of fun!).
Enjoy! Lots of stuff to see and do on Bermuda!
Phones/Wireless Devices: Doogee S90, Isatphone Pro, Amazon Kindle 3G, SkyRoam MiFi device, Karma MiFi device, AT&T Liberate MiFi device
Sim Cards: T-Mobile (Mint), AT&T (Mifi device or Kindle), Koko
Satphone: InMarSat
Broadband US Wireless Data: AT&T postpaid, Sprint (Karma Mobility prepaid)
Broadband International Data: SkyRoam
VOIP: Skype