Guam

Basics
Guam and the Northern Mariana Islands are territories of the United States that are not part of the 50 states. They share some things with the mainland, like the country code +1 and the currency is the US Dollar. But they have some significant differences with the mainland, such as having its own separate Visa policy, and more importantly its own independent phone networks.

The major US network operators such as AT&T or T-Mobile have no operations here, and consider these islands to be "international" with high roaming charges, or no roaming at all. Instead, there are three operators available on prepaid:
 * DoCoMo Pacific, owned by its parent NTT DoCoMo in Japan.
 * GTA
 * iConnect

DoCoMo Pacific
Docomo Pacific offers 2G service on 1900 MHz, 3G UTMS on 850 MHz, and 4G LTE on 700 MHz (band 17). Coverage map. Their prepaid SIM cards are sold in Docomo stores as part of a starter kit for $19.95 which includes $10 credit. Additional top-up vouchers can be bought in increments of $5, $10, and $20, and top-ups can be done by credit card as well.

Default data rate is $0.10 per MB. Plans are offered directed towards short-term visitors and billed by the day (24 hour period): This "unlimited" data appears subject to an unclear fair use policy, with throttling to 256 kbps on "excessive use" which seems to be several gigabytes.

In addition, there is a $1 monthly "E911" fee to be deducted from the balance. Service may be deactivated on reaching $0 balance, so better to keep a dollar or two on the account to prevent this.