It is what allows you to authenticate and connect to a specific carrier, i.e. ATT, Verizon, etc...

A "burner" phone, or any other cell phone, won't work without a SIM card, which isn't really a card. It's a small chip that you insert either inside the phone, or in a small slot accessible from the outside.

Of course the carrier has to offer service compatible with the phone. Pre-LTE there were different varieties of 3G that we’re not compatible with each other.
I would suggest price shopping directly with Tracfone, Cricket etc rather than buying cheap off Amazon as it can be tricky to get the right phone/service package when purchasing wireless ala carte.

Is what you want, either for at&t type GSM phones or CDMA verizon,sprint type phone.Then you need a plan with a company, they will provide a sim card which designates the company and the number assigned.If you are unsure how to go about it go to a local office of whichever service you want and they will install the sim for you and answer any questions you may have.But the phone you pick must be on their frequency..so if you want AT&T type service do not get a verizon /sprint,CDMA,type phone.

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If you are in a fringe area for reception you may be better off with a verizon/sprint type CDMA phone they have better reception in fringe areas

... and then add money to it when needed? I expect to use the phone very, very rarely. It isn't economical to have to renew every 90 days.

But thank you for your helpful information!

Some phones out there that actually go by a certain amount of minutes, you buy them more or less as you need them I am not sure which ones they are...a little research on walmart or amazon probably help you decide.I seem to recall one that you bought something like 100 minutes and had a year to use them and then bought more if needed

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Tracfone seems to have one that is 1500 mins for one year.. just check their web site

I've been using Tracfone for many years. I rarely need a cell phone but it does come in handy now and then. Such as, making the rare cell call It's also a good safety item to have.

Even spending the least I can for the call/data/text "packages" available they keep adding up...
5.26 GB DATA, REMAINING
6258 TEXT REMAINING
8012 MINUTES REMAINING

It's around $10 a month to keep it active. I finally just this year just purchased a year at a time card/plan. Not sure what to call it, maybe a "bucket of minutes" lol.

I can also use it for map/voice guided driving directions from my phone, again rarely needed. It was really handy navigating to a dental office in a much larger city than I live in. I may upgrade my iPhone, which is a lot easier without an expensive service fee. But, if you did make a ton of calls a different service may be better. Price then out.

https://www.tracfone.com

If I was not interested in tracfone I'd go "consumer cellular."

Disclosure, once upon a time I worked for years as a call service rep for Nextel/Sprint. Probably why I don't really like cell phones in general.

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I saw your question. You could purchase a "minutes" track phone card and keep it in your car/whatever and activate the minutes when you do need to make a call. Probably don't keep your same number though. I'm fairly confident that tracphone is probably the least expensive way to keep an active phone.