You will need an iPhone 6s, SE (1 or 2), 7 or newer, running iOS 13 or newer.
The GPS only Apple Watch can do Everything the cellular model can do as long as your iPhone is nearby, where nearby is somewhere in the same 3-4 bedroom home (assuming typical wood frame construction). And of course when the iPhone is with you when you leave the house.
https://support.apple.com/en-us/HT205547
The Cellular Apple Watch allows you to leave your iPhone at home while you go for a walk, run, bike ride, etc...
The Cellular Apple Watch shares the iPhone's phone number. This requires the iPhone cellular carrier to support the Apple Watch. The iPhone carrier charges a monthly fee for this service. In the U.S. the fee is $10 per month added to your iPhone bill.
iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:
https://www.apple.com/watch/cellular
Your carrier Must be listed by name in the above link. NVMO's that piggy back on another carriers network, unless explicitly mentioned, do not support the Apple Watch.
FYI: I have a GPS only Apple Watch, and I answered a phone call from my brother on my Apple Watch, and talked for about 30 minutes before I told him I was doing this all via my Apple Watch. He said he never noticed anything off about the connection. Of course then there was another 15 minutes talking about the Apple Watch 😀
