Texts not going through on watch that has cellular service

YOU CANNOT TURN OFF the iPhone!

The Apple Watch uses cellular LTE data and all phone calls are actually over VoIP (as in over the Internet). SMS/MMS TEXT messages use the traditional cellular voice network, and the Apple Watch cellular radio cannot access that network. So the Apple Watch has to use the Internet to contact the iPhone, and have the to relay all SMS/MMS TEXT messages.

If you turn off your iPhone, it cannot act as a relay.

Please go back and re-read the footnotes at the Botton of the website link I provided.

Yes, as long as the iPhone is powered on, and the iPhone has access to the Internet. Then the Apple Watch and the iPhone will use the Internet via iMessage on each device to relay SMS/MMS messages.

If one or the other is not able to access the Internet, or is not logged into iMessages, then no relaying of SMS/MMS can occur.

From the footnotes on the following website

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

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.

My phone’s battery died while on a bike ride, i.e. black screen and no signs of life. I tried to send texts through my Apple Watch to several friends. One went through but all the others failed. Why did that one text make it through when my phone was off? As I understand it, texts sent through the Watch require the phone be on.

I was able to call people through my Watch.

If the friend that got the message was using an iPhone, then chances are it used iMessage, which is an internet based protocol. The friends using Android phones, or that maybe have iPhones but are not signed into iMessage, would depend on the old cellular SMS/MMS protocols, that depend on the cellular voice network.

The SMS/MMS protocol depends on the 140 bytes unused on one of the old cellular voice protocol signaling packets.

The Apple Watch does not use the cellular voice network. It used VoIP (Voice over Internet Protocol).

The Apple Watch has an LTE data only cellular radio, so it basically ONLY talks to the Internet. This radio is smaller and less power hungry than a fully spec'ed out cell phone cellular radio that supports voice protocols, data protocols, and all the different radio frequencies. As you most likely to not want to have an Apple Watch the size of a cell phone strapped to your wrist, the smaller radio chip is a good thing.