VoIP is one of the traditional application scenarios for Mobile Ad Hoc Networks (MANETs) in settings such as emergency response. Ideally, VoIP would be transparent to the network type such that users would not have to worry whether their machine is currently part of a MANET or attached to the Internet. However, setting up a VoIP session using the Session Initiation Protocol (SIP) is difficult in MANETs since SIP relies on centralized components. In this paper, we show that the goal of transparent VoIP, for both isolated and Internet connected MANETs, can be achieved using SIPHoc, a SIP middleware for MANETs. We further discuss the deployment on laptops and iPAQ handhelds, and we look at various SIP interoperability issues. Categories and Subject Descriptors C.2.1 [Computer-Communication Networks]: Network Architecture and Design - Wireless communication General Terms Design, Experimentation Keywords VoIP, SIP, Ad Hoc Networks