To elaborate a little on the above (correct) answer…
You do not need to use the cellular capabilities of Apple Watch. So long as Watch is within Wi-Fi or Bluetooth range, the Watch will “leach” communications from the paired iPhone, including the ability to make and receive cellular calls. It’ll have all Watch functions, except the ability to send and receive telephone calls while remote from iPhone.
You should be able to initiate or receive a call from Wi-Fi without a cellular plan, if your carrier supports that with Watch and iPhone.
There is a second number associated with Watch when there is a cellular plan (all cellular-connected phone devices will have a telephone number assigned), though the telephone configuration works much like the party line telephone plans of years past, where a call to the iPhone telephone number will ring on both iPhone and Watch. You don’t need to remember or use the number assigned to Watch.
If you want Watch to be able to initiate and receive phone calls while located away from iPhone, then you will need an add-on to the iPhone cellular plan, as mentioned above. Otherwise, use it as a non-cellular-connected Watch, and there are no cellular fees associated.