Hi Ian
We've achieved this in 2 ways:
(i) using Flow. Create an activity template and embed in the journey, create an SMS account (we used Twilio), use Flow to trigger send by Twilio when the activity is created. It's a bit crunchy, but it worked for simple one-way SMS sends
(ii) Using Telesign custom channel - needs the Telesign custom channel to be downloaded from AppSource, instralled and then a bunch of configuration - but is a much more sophisticated solution (long/short codes, 2-way SMS, 2-step opt-in, etc)
Hope that helps