Choosing a carrier is really important since each carrier uses different technologies and offers a different set of features. The handsets they offer and their locality based coverage may also vary. Based on the technology, you might have to sign a long-term contract and be locked onto a carrier, or you might be able to switch among handsets.

The carriers in US can mainly be classified as National (Verizon, AT&T, Sprint, T-Mobile), Regional (MetroPCS, U.S. Cellular) and Mobile Virtual Network Operator (MVNO) type (TracFone, Boost Mobile, Virgin Mobile).

The main two technologies used by these are:

1) CDMA (Code Division Multiple Access) This technology is used by Sprint, Verizon,, MetroPCS and U.S. Cellular. The coverage of CDMA in US is high, and includes rural areas. It is thus very popular in the US; however the scope of this technology overseas is limited (no international roaming) and restricted to less number of handsets. Relatively new, this technology offers the following advantages: high communication security, high carrier efficiency (can serve more subscribers at a time), smaller phones, low power requirement, ease of operation for the network operators, and extended reach beneficial to rural users.

2) GSM (Global System for Mobile communication) GSM phones have a SIM (Subscriber Identity Module) card inside the phone that contains subscriber details and security information. SIM helps the carrier identify your phone. GSM phones can ideally be switched between multiple carriers by replacing with their corresponding SIMs. But it is likely that your carrier has installed some software 'lock' (more on that later) which renders your phone incompatible with SIMs of other carriers at least of a definite period. GSM supports international roaming and is a standard in many countries (Europe for e.g.).

Locking the phone by cell-phone companies

Some wireless carriers in the United States install software on the phones they sell to prevent that phone from working on another network, even if it uses a compatible technology. These locks often require consumers to purchase a new phone if they switch carriers. This is the case of most CDMA and GSM phones. This lockup may extend from a couple of months up to one or two years. The networks justify this by subsidising the cost of handhelds. Thus one must check whether the carrier under consideration has plans suiting him/her and whether it has coverage in his/her home, office and associated locality. If these conditions are satisfied, then one can benefit from the subsidy offered. Also check whether the carrier allows unlimited calls and data-plans or unlimited calls at off-times or 'rollover' feature (roll unused minutes to next month) etc.

A basic overview on popular carriers:

National carriers

They provide nation-wide coverage.

National carriers of the US are:

1. Verizon Wireless (CDMA) Presently, Verizon is the biggest wireless carrier in the United States. Verizon offers 3G & 4G services (limited to urban areas), GPS navigation, music store, dual-mode CDMA/GSM handsets and offers prepaid services. Smartphones from Verizon run on varied OSs including Google Android and Blackberry OS. Various unlimited call and data transfer plans are also available.

2. AT T ( GSM, previously known as Cingular Wireless) At & T is the second biggest wireless carrier in US. It has announced plans to buy T-Mobile recently. Features include smart phone support for all major OSs, 'rollover' plans, international roaming, support for latest phones soon after release, prepaid plans etc. 3G & 4G are limited to urban locality.

3. Sprint Nextel (CDMA and iDEN) Sprint merged with Nextel in 2005 to become the third largest wireless provider in the US. It offers unlimited calls and data-plans, a strong set of business-friendly features, extended 'off-time' plans, 3G & 4G in urban areas, multiple smartphone OSs etc.

4. T-Mobile (GSM) T-Mobile is the fourth largest carrier in the US features competitively priced and subsidized handhelds, unlimited calling and data plans, high customer service rating, 3G & 4G in urban area, family plans etc. On the other hand, mobile TV, music download and push-to-talk services are absent.

Regional carriers

They serve regional audience with home networks covering specific regions of the nation. Most popular ones are:

1. MetroPSC (CDMA) It is the fifth largest carrier in the US. Unlimited minutes are a feature of all its calling plans. It does not require the user to sign any contracts, and so phones are more expensive. Most plans have provisions for long distance and unlimited text messages, and web usage. It currently has a growing 4G network, but the 3G service is currently limited.

2. U.S. Cellular (CDMA) It is the sixth-largest U.S. carrier. Its native support doesn't extend nation-wide and so is not recommended for frequent travellers. Features include e-mail and messaging, 3G, push-to-talk services, GPS navigation, unlimited calling plans, prepaid plans and Android & Blackberry smartphones.

Mobile Virtual Network Operators (MVNOs)

These are companies that are provide mobile phone services but do not have their own licensed frequency allocation of radio spectrum and do not operate cellular networks. They use leased network space from national carriers instead. Popular MVNOs are:

1.TracFone (offers both CDMA and GSM) TracFone provides service as five brands: TracFone, NET10, Safelink Wireless, Straight Talk, and Senior Value Cell phone. These differ in the networks used and minute rates. Safelink is a govt. supported free cell-phone and free minutes service provided on income basis. TracFone offers prepaid services and doesn't charge activation fees. It offers only basic handsets and rates vary depending on the brand you choose.

2. Boost Mobile (CDMA and iDEN) It is suitable for budget-minded consumers. It requires neither contracts nor credit-checks and offers unlimited calls, messages, web browsing and push-to-talk service. But its coverage is limited.

3. Virgin Mobile (CDMA) Owned by Sprint Nextel, it offers prepaid services for the youth. Monthly plans include social networking services, GPS and YouTube apps along with unlimited nights and weekends.