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 that specifically includes support for Apple Watch, including the carrier activating the device on the plan:
Once the feature has been set up, your Apple Watch uses the same phone number as the paired iPhone.
Follow the instructions here to set up the built-in cellular feature:
- Set up and use cellular on Apple Watch Series 3 (GPS + Cellular) - Apple Support
- Use Apple Watch with a cellular network—Apple Watch Series 3 (GPS + Cellular) only
If you need assistance, then I suggest that you contact your carrier in the first instance.