Are you trying to send the SMS to your personal phone number? We have seen some carriers like Verizon block messages from IoT devices using our special non-geographic area code. You can call them to remove that block or test with a different phone number to see if that works better.
Sending data messages would be cheaper and faster, but of course you need to have some service somewhere that can receive those messages. It can be a little complex to do that manually and you may want to take a look at the documentation from ublox on that.
There are some examples on this forum though. Here’s one using UDP: