Re: iPhone 6 text message problem

I have the same problem. Basically, a temporary fix is to reset the network connection. 

Settings > General > Reset > Reset NETWORK Settings > Yes

This will reset your connect to the mobile network and hopefully help your text messages sync/send properly again. However, it only seems a temporary fix. I seem to have to do this once or twice a week!

Also, being connected to WiFi or BlueTooth doesn't seem to help where sending / receiving SMS messages other than iMessages is concerned.

Please note that when you reset network connection, your phone will need to reboot. I recommend saving any browser windows you have open, as well as ending any call you are in.

If I find a better solution, I will make an effort to post it here.