General:

The standard American telephone number is ten digits, such as (555) 555-1234. The first three digits are the "area code," which, in the past, indicated in what part of the country the phone was located.  At present, with the popularity of cell (mobile) phones, people in one area of the country might have phones with area codes from a different part of the country (since these phones were purchased there). If you make a call from a phone with an area code which is different from the area code of the phone being called, then you must always include 'area code' when you make such a call. Sometimes, the 'area code' must be entered even when the originating phone and the phone being called have the same area code. (In the case of calling between two phones that have the same area code, try the call without using the area code first; if you get an error message directing you to enter the area code before the number, re-call the phone by entering the 'area code', and then the 7-digit phone number).

Most often, you'll be given a "local" (7-digit) phone number only; don't hesitate to ask for the area code ("What area code is that in?")  If you are calling a number outside of your own phone's area code, you generally need to dial "1" before the "area code" plus 7-digit phone number. Note: In some area codes when calling locally you will need to dial "1" plus the "area code" plus the 7-digit phone number even if the phone you are calling is just across the street.

International calls are initiated by dialing "011", followed by the 'country code', the 'area code' and 7-digit phone number. If the phone you're calling from is a cell (mobile) phone, then the '1' is not required but the '011' is required. 

If you're originating a call from a phone in a hotel room or at a place of business, that phone will have its own specific dialing requirements which will usually include pressing '9' or some other number before the phone number you're calling.

Some numbers are reserved to provide a special use:

• 911 is the standardized number for emergency services (for example fire, police, and ambulance). You can normally call 911 for free from any phone (including cell/mobile phones) including pay phones (coins aren't necessary, just key in 911).
• 411 is used for telephone directory assistance. Fees are now almost universally charged for calling 411. As an alternative advertising-sponsored free directory assistance is available from a several providers including  Free-411 ((800) 373-3411) and Google ((800) 466-4411), typically after providing the city and business or person's name you're looking for, you'll have to listen to a short recorded advertisement before being given your number.
• "800" numbers (including 888, 877, 866, and 855) are "toll free" in that the user doesn't incur 'long distance' phone charges by calling the number. Callers should be aware that hotels and motels often charge fees for calling an 800 number and callers using cellular phones are charged for the "airtime" for the call following the same rate schedule as any other call. Callers to 800 numbers should also be aware that their telephone number is automatically being provided to the business paying for the 800 number even if the caller is blocking his/her caller ID. 
• Callers of "900" numbers are being charged either by the call or by the minute at a rate established by the company setting up the service. Legally, companies setting up 900 numbers are required to advise callers of the fee or 'per minute' rate for their call at the start of the phone call, with the caller being given the opportunity to end the call before the billing starts. Although originally intended as a method for companies to charge for informational services (weather, travel information, etc.) the use of 900 numbers has largely degenerated to "adult" services at the commencement of which call cost disclosures are sometimes omitted. Note: These calls can be very expensive!!

Be aware that hotels and motels frequently charge high fees for local, long distance, and toll-free number calls. There should be a card in your hotel room that outlines all the phone fees. If not, contact the front desk (reception) for information before making a call so that you don't have a surprise during checkout.

Cellular (Mobile) Phones:

Cell phone service is very good in urban and suburban areas, as well as along most stretches of popular highways (motorways). Service may be more limited in rural and remote areas and may not be available in some or all portions of national parks and similar natural attractions. There are four nationwide cellular service providers (in alphabetical order), AT&T (formerly Cingular), Sprint/Nextel, T-Mobile, and Verizon, as well as several regional providers. There are also an increasing number of "prepaid" providers who purchase network access from a national or regional provider and resell it to a customer, normally complete with a cellphone, with airtime being charged in advance on either a flat-rate monthly basis, or by the minute from a prepaid block of minutes. If traveling to multiple areas in the US, one item to consider before purchasing a prepaid phone is whether the phone can be used in the parts of the country that you'll be visiting. It's not uncommon for low-priced prepaid phones to only function in one specific area or region where the provider has contracted for cellular network access.

US cell phone service is split between two network protocols, with Sprint/Nextel and Verizon using CMDA and AT&T and T-Mobile using GSM and the associated SIM "chips" (which will be more familiar to visitors from many other parts of the world). GSM phone users should be aware, however that the US and other North American countries (Canada and Mexico) use different frequency bands, 800/850 MHz and 1900 MHz, for cellular communication than Europe and some other regions of the world where the 900 MHz and 1800 MHz bands are used. Users of "Quad-Band" cellphones will have no problems with their equipment whereas users of older "Tri-Band" phones may experience more problems with dead areas and signal dropouts (Europe-only phones also exist and naturally won't function at all in the US). Both AT&T (800/850 and/or 1900 MHz service, depending on the market) and T-Mobile(1900 MHz only) offer SIMs bundled with a prepaid calling plan if you wish to bring your GSM phone along from home. Both companies and local cell phone stores normally do not sell just the SIM card in their stores, and often do not sell to foreigners (no idea why!).

If you are considering purchasing a GSM cellphone in the US for later use at home, you should verify that the phone is "unlocked". US cellular service providers frequently offer "locked" phones which only function with their networks, for a significant discount or even free with a  two-year service contract. "Locks", which are code segments embedded into the cellphone's firmware, can be disabled using the software or a combination of software plus a special cable or other hardware but this entails additional expense or locating somebody knowledgeable in the process.

You can also purchase a pre-paid SIM card and or phone online as a cost-effective way to save some hassle once you arrive in the USA. There are many options out there. Be aware that if you see the “Simple Calling” SIM card, then there is an unpublished $0.50 connection fee for ALL calls – both incoming and outgoing. Those cards are mostly sold by a company called Telestial. Check out www.USASIMs.com– they look like they have a good offering and claim to ship not only to your hotel in the USA, but also to anywhere in the world.  If you live in Australia or New Zealand you can purchase AT&T SIMs from Mobi Passport. Another option is www.onlineabroad.com offering free shipping to any address in the world. They sell different US SIM cards on the AT&T network including free international calls and text for some of those cards.

A few important facts to note: There are no FREE incoming calls in the USA. All calls cost, whether they are outgoing OR incoming. Additionally, there are no separate rates for placing calls to landlines or cell phones; both cost the same amount. However, some providers, like Sprint/Nextel have limits on the number of calls you can make to 'land line' (non-cell phone/non-mobile) phones after which a 'per minute' surcharge applies.

Calling Cards:

Calling cards, which can be used for making long-distance or international calls at considerably lower rates than charged by a hotel, cellular phone company, or residential telephone service provider. Calling cards are commonly available from mini-markets, drug stores, or liquor stores as well as some gas (petrol) stations. Such cards are also available at some hotels, convention centers, and airports although caution should be exercised as some of this latter group of cards may only function with telephones at the facility. Finally international calling cards with highly discounted rates for calling a specific country (Mexico, the Philippines, India, etc.) or region (South America, Europe, etc.) can often be found in specialty markets or ethnic grocery stores catering to people from the country. 

In all cases, the cards function in the same general fashion, you call the access number listed on the card (which can be a local phone number or an 800 number), key in the card's PIN number (often concealed with a rub-off metallic covering to prevent somebody in the handling chain from stealing the PIN and using the card's allotment of minutes), and then enter the desired phone number.