I get an error "FeatureNotAvailable MSG-242" when I try to send an SMS from a sandbox account

I'm trying to send an SMS via the API from my sandbox account but I get this error.

I get the same error if I try via the "Test it out" button in the developers portal.

{ "errorCode": "FeatureNotAvailable", "message": "The requested feature is not available", "errors": [ { "errorCode": "MSG-242", "message": "The requested feature is not available" } ] }

I read all the threads in the forum but honestly cannot figure out exactly what to do.

It seems that everytime someone asked this a support person told them to try again after "enabling" something behind the scenes I guess?

Can you help me out please?