Watch will only text iPhones

SMS/MMS texting requires access to the iPhone. Even if you have a cellular Apple Watch with a carrier contract, the iPhone Must be powered on, and connected to the Internet so it can relay the SMS/MMS texts. The iPhone can talk to the Internet via WiFi or the iPhone's cellular connection.

For a cellular Apple Watch the iPhone does not need to be with you, it just needs to be accessible to the Apple Watch via the Internet

https://support.apple.com/en-us/HT205547

Read the footnotes on this webpage.

Hi, if you do not have a data plan you will only be able to iMessage. If your watch is connected to your phone with a valid data plan your watch and iPhone have the capability to text any number.