In terms of best practice and in some countries the law, you are not even allowed to follow up by sending bulk SMS messages with other offerings to the people who participated in your campaign, i.e.

Modern business requires companies to adopt efficient and smooth communication methods to achieve dominance in today's ...

From July, bulk SMS with unregistered sender IDs will be labelled “Likely Scam” to alert the recipient that the SMS may not be legitimate. • From October, SMS messages with unregistered SMS ...