Texts to other iPhone users aren't working under iMessage and iMessage is already activated.
Assuming those other users have an iPhone with iMessage activated, try turning it off and back on on your iPhone.
I tried that but it only works when I'm using wifi but not when I'm out, it sometimes ends up not working..
If wifi is not available, iMessage uses cellular data. Do you have a good cellular data connection? Do you have a cellular data plan?
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