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Buying in a different country
Apple Watch Series 3 (GPS + Cellular) is not locked to any specific carrier and has a built-in eSIM that copies information from the iPhone's SIM card.
The built-in cellular features require a carrier plan for your iPhone n your country of use that specifically includes support for Apple Watch Series 3 (GPS + Cellular), including the carrier activating the device on the plan.
If buying Apple Watch Series 3 (GPS + Cellular) in another country, then you would need to buy a model that supports the relevant LTE and UMTS bands for your intended country of use and your supported carrier there.
There are three different regional versions of Apple Watch Series 3 (GPS + Cellular), each supporting different LTE and UMTS bands, as detailed here - Apple does not currently list any supported carriers in Estonia:
Apple Watch - Carriers - Apple
If in doubt, I suggest that you contact your intended carrier to ask whether any given model can be used on their network with a plan that they may offer or plan on offering for iPhone that supports Apple Watch Series 3 (GPS + Cellular).
The built-in cellular features cannot be used when roaming. Also keep in mind that warranty service availability may be restricted.
Apple Watch Series 3 (GPS + Cellular) can be used without activating the built-in cellular feature, in which case it functions like a GPS-only model and can be used in conjunction with an iPhone 6 or newer model of iPhone (running the latest version of iOS) and a service plan from any carrier.
