Servers are with Gandi - which has a hosting API. So... SMS gateway via some shared hosting I have had for ages. This calls a script which figures out which server I mean and uses the API to shut it down.
I did used to use Twilio until they dropped international text messaging etc.
A shared hosting account which has a free (5 credits a month) SMS gateway included - I think it is reselling txtlocal.co.uk (who look quite good, but I haven't gotten round to trying them out directly yet).
It was kinda thrown together in a few minutes after Twilio flaked :)
I did used to use Twilio until they dropped international text messaging etc.