Most GSM cellular phones only have a single SIM card slot, but some offer the ability to install multiple SIM cards. There's no reason you can't use a single SIM card. Extra slots simply offer the ability to add another carrier.
Reasons for Dual SIMs
On a GSM phone, the SIM card holds all of the information the phone needs to send and receive calls and text messages. While most cellular carriers offer roaming, such charges can be high when you visit an area not covered by your carrier. Companies charge by the full minute and usually round up, so a call that lasts 1:01 is charged as 2 minutes of talk time. Many companies offer prepaid SIM cards. With a dual SIM phone, you can purchase a SIM card from a local company while traveling and use it in your phone without losing the ability to make and receive calls on your original carrier.
Installing Dual SIMs
The SIM cards on most GSM phones are installed under the battery. Depending on the device, the SIM card may go in a slot, slide under a clip or fit into a cage. The SIM holder or slots are usually numbered, and the SIM card for your regular carrier goes in slot 1. A few models have the extra SIM card slot on the outside. If you don't see a number on the port, check your phone manual to find out which is the default.
Activating the Card
Cellular phones are not connected to the network all the time while waiting for a call. The antenna eats up a large portion of the phone's battery, so most phones only check the network every few seconds. That's why the phone's standby time is much longer than the talk time. On some dual SIM phones, the phone will check one card for calls and then check the other. If the phone does not support this feature, you must switch the SIM card manually. Check the phone's Settings menu for a Call Settings or Call Options menu.
Drawbacks of Dual SIMs
Since a cellular phone only has one antenna for making and receiving calls, only one SIM card can be active at a time. For example, you can't talk on the phone on one SIM and receive a call on the other SIM. Any incoming calls received on the other SIM while on the phone will go directly to voicemail, even on phones that can check both SIM cards for incoming calls. The battery life on phones that check both SIMs at once is shorter, because the antenna is active more often.