I keep mine in a small zip-loc bag with a piece of tape on them attached to some paper saying the carrier, phone number and whatever other relevant info I need. Unfortunately if a SIM isn't used/topped up in a certain time frame, it is deactivated.
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Remove or switch your SIM cardMake sure you store your SIM card in a safe place if you decide to take it out, as any information saved on it – contacts, photos and text messages – will be lost if you misplace it. Wi-Fi and Bluetooth can be enabled without a SIM card.
An international SIM card or travel SIM card allows you to connect to a local network in various countries around the world. In this manner, you can avoid excessive roaming fees that your local operator or carrier may charge when you are abroad.
It's simply when your phone disconnects from your current carrier and attaches to a new network. International roaming happens when your phone disconnects from your current carrier while you're out of the country. Domestic roaming happens when it disconnects while in the U.S. (or U.S. territories).