<Li> Individual GSM, WCDMA, iDEN and some satellite phone devices are uniquely identified by an International Mobile Equipment Identity (IMEI) number . </Li> <P> Low - end mobile phones are often referred to as feature phones and offer basic telephony . Handsets with more advanced computing ability through the use of native software applications are known as smartphones . </P> <P> In sound, smartphones and feature phones vary little . Some audio - quality enhancing features, such as Voice over LTE and HD Voice, have appeared and are often available on newer smartphones . Sound quality can remain a problem due to the design of the phone, the quality of the cellular network and compression algorithms used in long distance calls . Audio quality can be improved using a VoIP application over WiFi . Cellphones have small speakers so that the user can use a speakerphone feature and talk to a person on the phone without holding it to their ear . The small speakers can also be used to listen to digital audio files of music or speech or watch videos with an audio component, without holding the phone close to the ear . </P> <P> GSM feature phones require a small microchip called a Subscriber Identity Module or SIM card, in order to function . The SIM card is approximately the size of a small postage stamp and is usually placed underneath the battery in the rear of the unit . The SIM securely stores the service - subscriber key (IMSI) and the K used to identify and authenticate the user of the mobile phone . The SIM card allows users to change phones by simply removing the SIM card from one mobile phone and inserting it into another mobile phone or broadband telephony device, provided that this is not prevented by a SIM lock . The first SIM card was made in 1991 by Munich smart card maker Giesecke & Devrient for the Finnish wireless network operator Radiolinja . </P>

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