Area code 671
Prior to 1997, callers from the rest of the United States had to dial 011-671- and then the seven-digit telephone number to reach Guam, similar to international calls. As international calling rates began to fall in the more competitive destination countries during the 1990s, it became a considerable financial burden for people on the U.S. Mainland to place calls to the U.S. territory of Guam, especially for those who were stationed in U.S. military bases there.
Cost of domestic calls 
After Guam was added to the North American Numbering Plan, the Federal Communications Commission (FCC) regulated calls to and from Guam as "domestic", hence dramatically lowered the cost of these telephone calls. This change also opened up national toll-free numbers (area codes 800, 888, 877, 866 and 855) to callers from Guam, which became very popular with the advent of pre-paid phone cards.
However, since no U.S. national mobile telephone carriers have a presence on Guam, cellular calls into Guam are not necessarily considered "domestic" or within the definition of "nationwide long distance".
Cost of international calls 
The FCC's regulation of Guam as "domestic" has no legal effect for telephone carriers outside the United States, for whom Guam is often not a competitive destination. Therefore, calls from other countries inbound to Guam can still be relatively expensive, especially compared to outbound calls from Guam since Guam is now open to the U.S.-based phone card market through toll-free access.
Within Guam, only the seven-digit phone number is necessary. When a person in Guam calls anywhere in the United States or Canada, one simply dials "1" and then the area code and phone number. For callers in the U.S. or Canada calling to Guam, one first dials '1-671', followed by the seven-digit phone number.
With a population of less than 200,000,  the number of prefixes on Guam is limited.
Here is a list of landline prefixes on Guam and their associated geographic locations:
- 3XX: U.S. Military
- 47X: Hagåtña, Agana Heights, Asan-Maina, Piti, Chalan-Pago-Ordot, Mongmong-Toto-Maite
- 56X: Agat, Santa Rita
- 63X: Dededo
- 64X: Tamuning
- 65X: Yigo
- 73X: Mangilao, Barrigada
- 789: Yona, Talofofo
- 828: Merizo, Inarajan, Umatac
- 969: Marianas CableVision's "MCV Phone"
See also 
|Guam area codes: 671|
|West: Pacific Ocean||area code 671||East: Pacific Ocean, 808|
|South: Pacific Ocean, country code +691 in Federated States of Micronesia|
|Hawaii area codes: 808|
|Northern Mariana Islands area codes: 670|