MO SMS is a message sent from a subscriber's phone to an application: texting "PORT 9876543210" to 1900 to start a number-port request, replying "STOP" to opt out of a list, or texting a keyword to a shortcode to enter a contest. It's what makes SMS a two-way channel rather than a one-way broadcast.

• Why it matters: without an MO-enabled shortcode or longcode, you can send but not receive — no keyword campaigns, no reply-based workflows, and no compliant opt-out handling for promotional SMS under TRAI's DND rules.