AFAIK the carriers require a double opt-in for this.
The first opt-in, which the Medium article describes, can be online with boilerplate language. But then you have to opt-in a second time by replying to an SMS sent directly to the device by the provider with language pre-determined by the carrier. The user has to reply YES to the text message, and you have to keep auditable records of these things.
If these 2 providers aren't requiring the second opt-in step, I expect they'll be kicked off the platform pretty quickly.