Basically, this is a question based on the proverbial 'chicken and egg' question.

We would like to know if you go by this sequence:
1. handset device you want to buy (let say N95)
2. network operators that offer N95

OR
1. network operator (let say Three)
2. pick a handset device that best suit your need

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What is network operator?
Company selling handset devices (mobile phones) at a subsidized price. Examples include: Optus, Vodafone, T-Mobile, AT&T, Singtel, Airtel, Three.

When should I choose Not Applicable?
If you are from countries (i.e. Malaysia) where handset devices are not locked. Basically, you can buy a SIM card from whichever network operator, put it into the card slot, and off you go.

The thing is that I am from Croatia, and here you can still by only SIM card, and go to any store and buy just mobile(not locked), but there is also possibility that you buy your mobile with SIM card at network operator and than it is locked.

Device each and everytime, provider doesn't really matter.

device! no matter provider!

Well in the United States you first have to choose between CDMA and GSM.

And, since not every operator is as good as the others and which is the best changes in different parts of the country you have to be aware of which operator you get.

If you want GSM you have to get AT&T or T-Mobile, otherwise your stuck with a CDMA phone from Verizon or Sprint.

Luckily pay-as-you-go phones are becoming more common with greater selection for the GSM operators and you can get a good phone without a contract, making it possible to get the phone you want and the SIM independently. But, it's still not as easy as in most countries.