Improper accounting of payment

Hi everyone, im Alex and in need of a legal advice with my problem. I have been a good customer of a well known telecommunications company for more than 2 years now. My problem started on march this year. To start with the details, i have renewed my service with the telco on december last year since im happy with my postpaid line. I availed a new plan that comes with a cellphOne unit. The plan requires a cash out for the unit, i was able to produce the needed amount of cash out that i have paid on December 16th, same day when i have got the new plan and cellphone unit. I was informed about the prorated charges since i have changed my postpaid plan before the cut off date of my bill. Come February i was worried to see that my bill was only 67 pesos. I knew there was something wrong that time so i called the customer service of the telco to ask about my bill. They just said that i overpayed the last bill or something. No one has informed me that the overpayment was the cash out for the cellphone unit. Then i received another bill amounting to 4,500+. After that i have been receiving text notifications to settle my overduebill to avoid disconnection. I kept calling to fix the problem with my account but i get no good resolution, the company just want me to pay the overdue bill(overdue just because of a mistake they have made). What is the best thing for me to do and what are the chances of getting my problem fixed if not, do i have a chance to win if i will sue the company. Thanks!

if the 4500 is the correct amount, then you are still obligated to pay for it. if not, make a written complaint

Hi! Thanks for your response. Yes the 4500 is correct but because they used the 4800 as a payment to the previous bill, they are asking me to pay for it again (for the unit w/c i have already settled). My only concern is i wouldnt have a bloated bill if they have posted the cash out for the unit on my account and not use it as a bill payment. I wouldnt have any of these problem if they have just properly accounted everything.