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If you mean, do you need a sim card to get cellular data, yes. But no it's not necessary, you can still use it without a sim card as long as you have your phone with you to get calls/texts for example.
 
You will need a "Wearable" plan from your carrier to fully utilize the Apple Watch Cellular features. However, if you have your iPhone with it then you can utilize Bluetooth and or Wi-Fi to make and receive calls, send and receive Txt Messages and use data.

Dave
 
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