Is That Your Final Answer?
Not content with the reality that your customers have specified they don't want unsolicited contact from you, you have the timerity to require all telephone enquirers to your customer care centre to explicitly state that they still do not want you (or approved third parties) to contact them with occassional "marketing" offers.
Do you think they've suddenly had a change of heart or do you assume that they will be more interested in discussing what ever issue caused them to call in the first place and will forget to confirm their position?
That might increase your mailing list, but it will only do so with people who have already shown themselves to be opposed to spam and who are likely to greet your offers with indifference or, worse, annoyance. Tricking your customers is the dumbest marketing of all.