In order to use the phone parts of FONA you will need a SIM card. Luckily, there's a phone store in every town in America! You can get a pre-paid or post-paid SIM but we kinda like the pre-paid kind.

A 2G or 3G Mini SIM card is required to use the module. Nearly any cell phone shop can sell you a SIM card. Luckily, every SIM we've seen is 2G or 3G so you are good to go!

The FONA 800 and 808's use a 2G SIM only. Make sure you're using a SIM that can do 2G. In the USA, that's a T-Mobile or reseller

The FONA 3G can use either 2G or 3G. These modules will only work with AT&T in 3G mode. Unfortunately, AT&T is deprecating its 2G network, so you will only be able to use this module in 3G mode with AT&T. If you want to use it with 2G, use T-Mobile

MicroSIMs won't fit - so make sure its a "Mini" SIM. Sometimes these are just called plain "SIM" cards since the huge-size SIMs are rarely used. Mini SIMs are 1" x 0.6" / 25mm x 15mm. these are by far the most common size.

Adafruit now sells the TING SIM card, a 2G+3G SIM that works great with all FONAs, and has a great billing system as well, where you only pay what you use!

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You do not need to bring in or show your FONA to the Cell Phone store. Just tell them you need a Mini SIM for a cellphone and it's at home.

Some suggestions from FONA users!

I've had good luck using the FONA with a SIM from Walmart. Their 'Walmart Family Mobile' is a T-mobile reseller and you can pick up the SIM starter in store pretty easily . The service isn't the cheapest, it's $25 for the SIM and $30/month for unlimited talk+text or $40/month for unlimited talk+text+data but there's no contract as its all prepaid -tdicola
Just want to let the Adafruit team know that it is possible to get pre-paided minutes on T-Mobile and its a pretty good deal. I was paying $100 for 1000 minutes (10c a minute 20 a sms) over the course of the year. For me it in my usage came out to about $8 a month. Have found this is something that you have to ask for at the store. No evidence in advertising or website -Paul B.
Telna has a good deal you may want to point people to for FONA. I used them for a previous GSM project and they worked great. $20 per year, includes 1000 free txt messages and a bunch of other nice features. I had a lot more issues with T-Mobile (Their cards go inactive if not used in a month and you have to call to reactivate) -Tyler C.
I pay 2 EUR (a bit less than $3 USD) for 2 hours, unlimited SMS and 50Mb of Data at http://mobile.free.fr/ -KTOWN
There are specialty "machine 2 machine" SIM sellers that have plans that are tuned for the short bursts of usage used by these kinds of modules. Check out these guys for example, you can choose data, voice/sms or a combination for a pretty low price!