The goal of this master thesis is to examine the opportunities a customer has to object to controversial bills. Vitally important is the right to appeal against a bill, which is reflected in the Telecommunications Act 2003, as well as all General Terms and Conditions of mobile operators. Thusly, the first part of this thesis deals with the General Terms and Conditions law, especially with the limits that apply to the individual clauses. Subsequently, it is important to clarify what constitutes a bill needs, its obligatory contents and which rules for access apply. Based on the first two chapters, the third chapter deals in detail with the actual topic, the bill opposition. First, the rules of § 71 TKG 2003 are presented in detail and compared with typical used bill opposition clauses. However, the real difficulty is found in the legal interpretation methods, that are used to determine which fees can be challenged with the help of a bill opposition. Following this, the most important recorded cases are presented and arguments against payment obligation are identified. The last chapter describes the settlement of disputes in front of the regulatory authority according to § 122 TKG 2003, which may be introduced after a failed attempt to enforce one's claims against the provider.