We have a Moodle environment with the following configuration:

• SMS is enabled in General > Messaging > Notifications
• An SMS gateway is defined but disabled. We tried this with AWS and with the contrib Twilio plugin.
• User A is enrolled as a Teacher in Course 1
• User B has a phone number in their profile but does not have SMS notifications enabled.

We've observed the following scenarios:

1. User A enrolls User B as a Student in Course 1.
   1. User B is enrolled, but User A does not see the success message and the page reload is not triggered.
2. User A sends a message to User B.
   1. The message is delivered, but User A receives a script error.
3. User B removes their phone number from their profile, and then User A sends them a message.
   1. The message is delivered, but User A receives a script error (similar to but different from above).

It seems like there's insufficient error checking around the SMS subsystem.