Can’t send text/SMS messages on Apple Watch 4 when iPhone is not near can only send imessage

SMS Text Messages REQUIRE a cell voice radio. The Apple Watch is a DATA ONLY cellular radio device, and as such it does only does voice over Internet and does NOT have SMS capabilities. So it needs to be able to find the iPhone and ask it to send SMS messages.

The same is true for a GPS only Apple Watch.

IMessage works because it is internet data only service.

So no iPhone, no SMS Text.

If the Apple Watch can find the iPhone via Bluetooth, WiFi or a cellular data connection, you can send SMS text messages.

See this support document, including the footnotes at the bottom as far as what you can do with the Apple Watch without its paired iPhone around. How to use your Apple Watch without your iPhone nearby - Apple Support

The constraint is not “Apple or non-Apple” so much as “internet data or cellular telephony data services” listed in Footnote #1 in the support document. This limitation is outside Apple’s control and linked to how networks handle of old style accessory services like SMS.

The hookup for voice calls seems to involve remapping the (hidden) number tied to the eSIM so the network forwards incoming calls to the iPhone and the watch. This is not too difficult, call forwarding for voice has been around for ever, but there is no standard for forwarding SMS etc. This constraint doesn’t apply to iMessage which is routed through Internet data rather than SMS using spare bandwidth on a scheme which originated with the design of the original GSM service.

The Apple Watch GPS+Cellular is NOT a cellular data only device, it makes calls when you are away from your iPhone (and if you know the billing phone number for the Watch you can actually make a call directly to it. With my phone on AT&T I have found my iPhone is not with me but it is turned on I can send and receive SMS texts. And yes you can find it via cellular connection.