Error in billing - need activation date of phones

I am also trying to find my activation date. I bought 2 new phones on the same day last year under a buy one get one 1/2 off promotion. Unfortunately I have not received any credits on my bill. After looking at my bill I realized the number of installments are 11 out of 30 and 9 out of 30 on the other phone. I bought the phones and activated them on the same day. Why did this occur and how can I provide evidence to get the promotion applied? I do not have a receipt.