Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Whether the offer is legitimate really depends upon
1. who it claims to have made the offer, and
2. whether it was really from that offering-company (as opposed to a third party, or a forger who is trying to 'phish' information from you or offer you a scam etc.)
So you did well to question its legitimacy. The best way to confirm the offer (or fake / scam etc.) is *not* to follow any link in it, *not* to do what it says. Instead, look at the reputed offerer's regular website, or call their regular number, asking about the offer. If it is really their offer, they will know about it; if not, they will advise you it is a scam and you may delete the text message with perfect cheerfulness, having avoided certain possible hazards.
E.g. if it claims "Verizon will guve you unlimitted sevice for ten dollurs call 1-888-SCA-MMEE" then research Verizon's number, call Verizon (not at that 888 SCAMMEE number but Verizon's regular number) and ask Verizon about the offer.