IP telephony has become one of the most successful applications of the peer-to-peer paradigm. However, providing a scalable peer-to-peer-based distributed telephone directory for searching user entries is still an unsolved challenge. In this paper we present the Extended Prefix Hash Tree algorithm that can be used to implement an indexing infrastructure supporting range queries on top of Distributed Hash Tables. The algorithm supports hierarchical keywords, and is especially designed for situations where a few keywords are very common while others are very uncommon. The design of our algorithm perfectly matches the requirements found in a distributed phone book scenario. We point out the difficulties that previous algorithms had with data that is distributed that way, and evaluate the performance of Extended Prefix Hash Trees with a real-world phone book. The results can be transferred to other scenarios where support for range queries is needed in combination with the decentralization, self-organization, and resilience of an underlying peer-to-peer infrastructure.