The past five years have witnessed a massive increase in the use of mobile cellular broadband devices such as smartphones. These devices often contain multiple radios permitting WiFi, WCDMA and HSPA access in a single device. Their increased processing power and multimedia capabilities have made them attractive for use for new services such as mobile Voice over IP (VoIP). We provide an empirical analysis of VoIP performance over WiFi, WCDMA and HSDPA radio interfaces on a typical high-end Smartphone, with measurements of VoIP quality metrics such as end to end delay, packet loss and jitter. We observe that the best performance in terms of mean opinion (MOS) scores was obtained in WiFi environments, while the poorest was recorded in WCDMA networks. We find that VoIP codec processing delay in these mobile devices is the most significant contributor to end to end delay, and that optimization in this area will provide the greatest improvements to mobile VoIP voice quality.