Having the same problem here. Still actively looking for a fix.
Below is a summary of what I done so far. Perhaps one of these step might work for you?
As I'm not from the states, I didn't try the AT&T or Verizon steps. You might like to do that if u are based over there.
Check Signal - Done
Reset Network Settings - Done
Check Carrier - Done (Am Able to send Text via old Iphone 4)
Verify that every phone number in your contacts list includes an area code. (Done)
When sending to existing contacts, ensure that your contacts include the full phone number, and not just the information needed to dial locally.(Done)
If you have recently ported your number from another carrier, please contact your current carrier to verify that the porting process has completed successfully. (Done: This is an old carrier, it's not an issue as I am able to use the same sim card to send text via my old iphone 4)
Perform a full restore back to factory original setting (Done)
Nothing seem to work for me at the moments, I wonder if anyone else is having the same issue