Adding phone service?

When I try to add a phone service to my bundle, it tells me that I have the best package available online, or the promo code turns red and says it is invalid - I have tried multiple times to add phone service, but it won't let me, no matter how I try to back myself into it. I would like to add phone service, if at all possible. Would it be 10$ or do I have to pay more because the promo code turns red if I try to add that way?