Try to find the balance, and then during the day absolutely do not use the phone for any paid services (calls, messages, Internet). On prepaid fares monthly fee for the services, if any, often is deducted in equal installments every day. The next day, again check the balance - if it changed? If Yes, the monthly fee for any set of services available. Multiply the difference between yesterday's and today's balance by 30, and you will see how much you have to pay for these services per month.
Note that some operators a subscription fee for some services is written off once a month even with a prepaid rate. Some third-party content providers write off the funds for subscription to their services every few days.
Call the support service of the operator. Ask what services with a monthly fee you signed, and does the subscription fee of the tariff. If necessary, discard some of these services or change the rate.
If the subscription fee is charged for unlimited Internet or unlimited calls within the network or on your favorite numbers or unlimited sending of MMS messages, the service cost is low, and you actively use it, to abandon it is unwise. Without such services, you will surely spend much more money than her (you can even try to calculate how much more).
Some operators find out the rate and the list of connected services can be found via SIM-menu or a personal account on the website. Will you stay to watch the parameters of the rate and the cost of each of the services on the website or ask them from a consultant.
If every few days from the account your phone is charged an unusually large amount, although you never called and sent messages, perhaps you subscribed to the service to third-party content provider. Sign you up unknowingly to children or relatives, having bought on advertising on questionable sites and not bothering to pre-acquainted with its rules. Also it could especially make strangers who you once gave the phone to make an urgent call for a couple minutes."
Call support operator and ask the counselor to inform you, in favor of what the content provider was charged. If the Advisor does not give you the phone content provider, try to find it on their own name using search engines. See a counselor the content provider and ask them to disable all subscriptions. Sometimes it is necessary to do it yourself, by sending the special message number with the command "STOP" or similar.
If a subscription has not stopped, again, call support operator and ask him to enable service blocking payments to content providers. Different operators it is called differently. Note that while you may lose the ability to use mobile transfer.
Some operators impose paid services in advance by connecting them immediately when purchasing the SIM card. This can be, for example, Voicemail or replace the beeps with melodies. Subscription fee for such Intrusive service starts to be charged immediately, but after a period, usually equal to two weeks. If you find that subscribed to this service, be sure to discard it by asking the consultant to disable it or tell you how to do it.