I have also the issue with the sms sending. I have a 10III and with the OS 4.6.0.15 I had the sending issue when the VoLTE was active. Now in 5.0.0.55 I have the issue also when the VoLTE is not active. 
With what I have found out, there is 3 ways to “solve” the sms issue.
1: Flashing the 4.6.0.15 on the phone again. According what is installed on the phone this can take a long time.
2: In my case it is possible to receive sms, that’s good, so when sending it is possible for me to send using mms. So the person you are sending mms to is thinking why pictures every time.
3: I have notice, when the settings under the network mode is changed to 3G it is possible to send sms.
Hopefully the issue will be solved with the next update. Or, is there anyone who have a fix when knowing the above.