twitter had/has so called "zero rated" deals with most carriers, cut way back when. They dont(at least they didnt used to) pay anything per SMS, for most carriers.
However, the infrastructure to go direct to that many international carriers is/was quite complicated, from both a technical & ongoing business management PoV (carriers are awful to deal with in both categories). i imagine the savings from no longer having to maintain either are quite substantial.
https://stackoverflow.com/questions/3668372/how-was-twitter-...