It should be a sub-account level option to completely disable texting capabilities.
An example of a use-case is
for new sub-accounts, or sub-accounts that have not yet been registered with Twilio. The sub-account still needs to be able to use phone funtionality without concern of accidently trying to send text messages.
This would also mean that any SMS automations would simply be skipped and there would be no accidental expenses.