Network Function Virtualization (NFV) is an emerging solution that aims at improving the flexibility, the efficiency and the manageability of networks, by leveraging virtualization and cloud computing technologies to run network appliances in software. However, the “softwarization” of network functions raises reliability concerns, as they will be exposed to faults in commodity hardware and software components. In this paper, we propose a methodology for the dependability evaluation and benchmarking of NFV Infrastructures (NFVIs), based on fault injection. We discuss the application of the methodology in the context of a virtualized IP Multimedia Subsystem (IMS), and the pitfalls in the design of a reliable NFVI.