Why the channel exists
A missed call asks almost nothing of the person. No data connection, no form, no typing, no cost. In markets and audiences where any of those is a genuine barrier, that matters, and it is why the mechanism has persisted long after forms and messaging became widely available. It is a hand raise, and it is very good at being a hand raise.
The trade is information. A form tells you what someone wants, a WhatsApp message gives you their words, and a missed call gives you a number and the campaign it came from. Everything else has to be established by a person, quickly, which is why the follow-up process rather than the campaign design determines whether the channel earns anything.