The recent widespread deployment of wireless LAN technology raises the question of how a mobile telephony system might instead be architected to use wireless LAN access points and the Internet to achieve similar services. In this paper, we examine an end-to-end architecture for mobile telephony, with a strong focus on endpoint issues. We have designed, implemented, and have experience using devices we call Internet Mobile Phones or IMPs. The IMP system provides encrypted wireless voice communication over 802.11B LANs. IMPs run Linux on a lightweight single-board computer running customized voice over IP software; data is encrypted with 128-bit Blowfish. The paper reports on our design decisions and the resulting implementation of the IMPs, with sufficient detail to reproduce the devices. We report our experiences with using them for several months in a laboratory environment, and close with proposals for future experiments to investigate scale and extensibility.