Grandfathered Simple Choice

I have been having the same problem with my grandfathered plan that was $10 for each additional line. I have been getting a lot of run around. Each time I call, the representative gives me a new spin. Now they are telling me that they never had a plan where the 1st 2 lines are $80 and each additional line is $10. At one point I had 5 phone lines on my plan but went back down to 2.  I know what I used to pay and they are now basically telling me that I am lying. I have had this plan for 7 years now. Since I added another phone line some months ago they have been charging me $20 for the line! Please let me know if you have gotten a solution to this problem.