I just ordered an Apple Watch 3 from my cell carrier. After the order was placed they said my watch has a new phone number. I was under the impression that the watch would simply use my existing cell

Hi

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:

Apple Watch - Carriers - Apple

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.

You're welcome! 🙂