c) grey route outbound sms. Even cheap US plans tend to have 'unlimited' sms, sometimes even to selected foreign destinations. Sometimes carrier billed SMS is cheaper than aggregators (but not too often) or may have better routing to difficult destinations.

Yes, I can definitely see that being plausible, particularly if they've gone to the efforts to make software tooling to spread out the outbound SMS volume around many different SIM and self-rate limit their volume, to avoid getting cut off, rate limited, or account banned.