What’s the cheapest add a line deal? We told you about T-Mobile Even More Plus Plans and now they are having a $5 add a line promotion – the cheapest add a line deal among all major carriers. While everyone else is charging you $9.99 for an additional line in a family, T-Mobile has stepped up the game by offering a cheap add a line plan. What’s included in this promotion is that calling between family lines are free so that you don’t have to worry about running out of monthly minutes because of added lines. Of course, you don’t need to add 5 lines at the same time just because it’s the cheapest add-a-line plan available now. Do it as needed. Good luck.