This post demonstrates a potential fix for SMS problem with Samsung Galaxy S5 as indicated with an error prompt saying, 'message blocking active.' If you have encountered the same problem while attempting to send a text message, then you can use the following methods to fix it.
This error usually occurs when your SMS message recipient enabled message blocking setting with their carrier. In this case, ask your recipient to verify if message blocking setting is enabled with his/her carrier beforehand.
Next, delete message threads or messages from your phone and/or SIM card memory and try to send a text message again. If you still cannot send, reboot (soft reset) your phone. After the reboot, try to send a text message to your own number to see if you can already send and receive texts.
If problem persists, please contact your network provider and check for any possible outages that might have affected messaging services in your location. You may also contact your carrier's tech support for further troubleshooting assistance.
Another possible reason as to why you can't send a text message to a specific person is because your number is probably blocked or marked as spam. You can have that person check their phone's settings menu to verify if your number is accidentally listed as blocked or tagged as spam then have it removed from there.
Note: Message blocking feature on the Galaxy S5 is accessible depending on your carrier.
The above solution is also applicable to other Android smartphones.