I was promised a third line for free and didn't receive it.

I have been a customer for 10 months and I recently added 3 new lines. I was told that the 3rd line would be free. When you go on there website it shows that it is 160 per month for 4 lines on magenta. That means that it's $40 per line. I was under the impression that my bill should be 120 because 160-40= 120 but according to them I need to pay 140. Which only gives a $20 discount. They say that the deal is that they are giving you 4 lines for the price of 3 ($140) to skip out on that extra $20. Customers would actually get a larger discount if they went by the price they show online and subtract the value of the line from that.it's just frustrating because how would a customer know that based on info given on the website. On top of that when I inquired about the lines they didn't even explain it correctly. I still don't understand how they calculate the cost of each line but I think they decrease each line a different amount as you add more lines. So if they make the last line $20 they can give it to you for " free" which makes it seem like they honored the promotion but in reality they owe you some more money. That has to be the only way that a line on a $160  4-line plan is actually only worth $20. They decided to give me some credit but that's after going back n forth for so long and demanding that I be compensated. I am still unsatisfied with what they gave me but I went ahead and accepted it cuz I was tired of arguing. I had another issue because I was trying to upgrade and use promo for iPhone 13. A rep told me that I wouldn't have to pay anything to get the promo and  to upgrade  and that they would give me $400 credit. Well that turned out to be a lie because in order to upgrade and get the promo you'd have to pay off your previous phone. So this ordeal turned into a 3 day argument because they didn't want to honor what the rep told me at first. Mind you, I had to speak with 3 different people before the truth was actually unveiled. While talking to those people and using the T mobile website I was given FOUR DIFFERENT AMOUNTS I would have to pay in order to get the promo. I understand they have policies but if those policies are misleading then they need to correct it.And if employees don't do their job correctly then they should have no problem adequately compensating the customer and honoring the agreement made. It sucks because metro , and sprint are now apart of T-Mobile. I originally wanted an account with sprint but was automatically enrolled with t mobile so it's kind of like we're trapped. There's not many good options for phone services now. Verizon is way too expensive   and other services aren't well known so I guess T-Mobile can treat customers however they want. I like the actual phone service and some of the perks T-Mobile has but their customer service needs to get it together.