What do SIM cards do for travelers?

I talked with another traveler about SIM cards but I don’t understand how they work. If I bought a SIM card online for example, I can make and receive calls in another country?

Is this how it works? How do I know if a certain SIM card online will work in a different country? Or work with my phone?

Example:

Home country: Canada
Country visiting: United States
Phone: Iphone 4
SIM card found online:

Thanks in advance!

The Idea is that you buy a sim card for a mobile network of the country you are VISITING. That way you can use your phone to make and receive calls in that country without racking up exorbitant roaming fees from your home mobile provider. In your case, that Fido SIM card won’t do you any good in the US, since it’s a Canadian mobile network. The best thing to do is to arrive in the US, buy a compatible sim card/pre-paid service from one of the US carriers (you can find them in most malls and Wal-Marts), insert it into your phone, and activate.

Additionally you need to unlock your phone from your carrier. This is a carrier (AT&T, etc.) initiated action and there should be instructions on its website if you haven’t done it yet.

Coincidentally, the NYTimes had a great article today about some of the best rates to common countries and a little about the technical aspects of swapping the SIM.

I will also add that I don’t know about visiting the US, but for every other country, I just buy a card at a local retailer after arrival in the country. Travel stores, Tobacci, mobile shops are plentiful and obvious to find- but don’t do it at the airport, they are 10x the price! The NYTimes way overestimates the amount of data needed! 200 Mb is far more than I need for a week of restaurant reviews and an occasional google map lookup- but I do turn off all other services or restrict them to wifi only (like facebook and photosharing, etc.).

Besides needing to be unlocked by your carrier, the sim card needs to be accessible by the user. Some phones do not have their cards accessible.

adding to what has already been posted.

I always tell people that a sim card is a network interface device, between the phone and whichever network. it is what is linked to your phone number. this is why if you switch the sim card between compatible devices (that is sim card size), whichever phone now has the sim if someone calls your number it will ring that device.

About international usage, first check with your carrier. i know certain AT&T plans one can get mexico or mexico and canada roaming at no additional costs. but one either needs to provision online or contact at&t. other carriers might have similar deals.

About unlocking phones, if your carrier doesn’t have an official unlock website, contact your carrier. some offer unlocking, others don’t, and others unlock only certain phones and still others only will unlock for international travel. i know the rules for at&t say the phone has to be fully paid off or out of contract and the account has to be in good standing, the phone cannot have been reported lost or stolen.
lastly allow enough time, often the permission to unlock the phone is quick, typically it can take up to two business days and depending it can take longer. once given permission by your carrier. and typically, non iphones when you input the foreign sim card will ask for a unlock code. iphones need to be connected to itunes, to backup, restore (erase) and reactivate. do not wait until one is at the airport to call your carrier. and yes this means that if you are using a foreign sim you will also have a applicable phone number.

Some countries are tricky, though. In South Africa I had to ask the guest house owner to get a SIM for me. I’ve never successfully acquired a SIM in India.