Apple Series 3 I just bought it today and it allows me to send and receives text to other iPhone users in my contacts perfectly However when?

I try to text my sister from the device who is not an iPhone carrier it will say not delivered or message send failure on the watch HOWEVER since the phone mirrors the watch it will successfully send it through on my phone, just not the watch how can I fix this, I have went through the watch settings because I thought I set the notifications up wrong but everything is fine because if it wasn't I wouldn't be able to successfully send or receive from other iPhone carriers, help?!

It's an Apple product, of course it's not going to play nice with all the other standards based phones in the world!

For the record there's no "iPhone carriers". There are carriers that of course offer the iPhone but that doesn't make them that since the iPhone (if unlocked like any other phone) can work on any GSM carrier worldwide. I assume you meant to say that she doesn't own an iPhone, so a non-iOS device in essence.

I'd suggest checking over the settings again. I have an iPhone/Apple Watch & have no issues texting non-iPhones/iOS devices through it.