Can a cellular Apple Watch temporarily fill in for an iPhone?

The watch can certainly sub for your phone while it's in repair. I use mine that way often when my phone is at home and I'm out & about.

You will likely need to learn to use Scribble to send text messages.

M1 Mac mini, macOS 12.0.1; Watch, series 5 GPS/cellular, watchOS 8.1; iPad Pro 2nd Gen, iPadOS 15.1; iPad Air 2, iPadOS 15.1; iPhone 12 Pro, iOS 15.1; TV 4th Gen, tvOS 15.1; HomePods, audioOS 15.1 and Airport Express w/ Airplay 2

Dah•veed wrote:

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I didn't realize the cellular watch had this disadvantage.

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

At the bottom of this link, footnote #1

1. To receive SMS, MMS, or push notifications from third-party apps on your cellular Apple Watch, your paired iPhone must be powered on and connected to Wi-Fi or cellular, but it doesn't need to be nearby. You also need to be signed in to iMessage on your iPhone.

The Apple Watch uses a cellular "Data" only radio. It does voice phone calls using VoIP (which the carrier must support).

However, SMS/MMS depends on the traditional cellular voice network, where is uses unused bytes in the voice network's signaling packets. An LTE Data cellular radio does not use these signaling packets (it is all TCP/IP), which is why the Apple Watch needs to ask the iPhone to do the SMS/MMS heavy lifting.

You will be able to use iMessage, but SMS/MMS Texting requires the Apple Watch be able to communicate with your iPhone over the internet.

If you are frequently TEXT'ing with Android users, this will now work while you iPhone is in for repair.

I guess that I'm lucky that everyone I know wouldn't be caught dead with an Android phone!

I didn't realize the cellular watch had this disadvantage.