SMS is the acronym for Short Message Service, which as we know is a service component of phone, Web, or mobile communication systems. The acronym and the spelled-out term are nouncount nouns that refer to both the type of service itself as well as to the message it renders. From a grammatical standpoint, therefore, the term SMS by itself has no plural form; we can't say "300 SMSs," "300 SMSes," or "300 SMS's" (this last is the so-called apostrophe-s form, which certainly can't be considered a valid plural form nor even a proper possessive form). Only specific countable items that the acronym SMS modifies can be rendered in the plural, as in the noun phrases "SMS capabilities of state-of-the-art smartphones," "the increasing frequency of dropped SMS transmissions," and "numerous SMS signals of unknown origin."
