Subscriber identity module cards do not play a role in a cellphone's reception quality, making it impossible for them to cause bad reception for the device. However, in some situations a SIM card affects a cellphone's ability to connect to its provider's network, but not because of bad reception.
SIM cards are stamp-sized microchips that store some personal information, including account number and phone number, relevant to the cellphone and its user. The SIM card provides this information to the device's processor, which, in turn, sends it to the provider's wireless network. This information identifies the phone to the carrier, giving the phone permission to access the network. This information is transmitted through the phone's transmitting components, which use radio waves to send and receive transmissions. These components do not rely or involve the SIM card in any way for reception, except for accessing the necessary data needed to connect to the network.
In some instances, a SIM card can become old or corroded, making it incapable of establishing a connection to the cellphone's processor. In this situation, the data that identifies the phone to the network fails to transmit, making it impossible for the phone to connect to the carrier's network. This occurrence has nothing to do with the phone's reception functionality and everything to do with the SIM card's functionality. If the SIM card was designed for use in phones that access the 2G network and was transferred to a newer device that is capable of accessing the 3G or 4G networks, the card could be incompatible with the phone's processor. This could cause the phone to fail to connect to its provider's network.
Replacing a cellphone's SIM card helps you determine if it is the cause of your device's inability to connect to the provider's network. However, replacing a SIM card does not improve your phone's reception quality. Bad cellphone reception is more likely due to a stormy weather, network traffic or proximity to a cell tower. Using a cellphone in certain locations -- such as buildings that contain a lot of radio equipment -- can cause you to have poor or no reception while using the device.
SIM cards also support contact information contained in your cellphone's phone book application, making it possible to transfer this type of data from one mobile device to another. Only cellphones that use GSM technology require a SIM card for a device to connect to its provider's network. CDMA phones do not need a SIM card to access their provider's network. CDMA phones store the data needed to access the network internally.