I just got an Apple Watch 3 and I had connectivity issues when I first got it (wasn’t sending/receiving ANY message or calls unless iPhone was turned on...defeats the purpose of an Apple Watch with cellular). I talked with T-Mobile tech support last night and we turned off bluetooth & wifi, reset the messages settings on my phone, restarted phone/watch, and tested everything and things finally worked.
However, a few hours later I turned my phone off again to test everything, and everything was working EXCEPT SMS. I tried texting a friend with Android, and nothing went through, but I did get iMessages and calls when my watch was connected to cellular without being paired with my phone.
I just got off the phone with T-Mobile again, and basically they said that this is just a known issue (from searching these boards, I see a lot of posts about this from 2 YEARS ago, but would’ve assumed it would be fixed by now). Is there any other solution? I don’t see the benefit of paying for cellular if I can’t send or receive SMS messages - I don’t like Android, but that doesn’t mean my friends, family, and coworkers don’t have Android devices, ya know??