That really isn’t how that is supposed to work… I’m surprised and shocked it is this way for campaigns.

In all of my experience as an engineer, all SMS gateway apps will automatically remove the number when you text stop.

The reason is because it’s rife for abuse should you let the organization handling the messaging handle stop themselves.

IE, muting the conversation or making continuous messages to it will cause the sms gateway number’s reputation to fall which can affect their business