Support sending SMS messages

I'm not sure why the test on line 25 of SendSMSTest fails (in the log I see "expected (SMS nuntius), got (SMS nuntius)"), but I think we can deal with it in a followup
Do we know that this is correct? If this is just a stub, we should warn about it. Otherwise, it's worth a comment explaining why we know that the message will fit into a single segment regardless of its length.