Opportunistic augmentation denotes connecting a personal mobile device to another device to gain a transient advantage for the user. For example, a mobile phone user might borrow a large display and keyboard from a desktop personal computer. This uniquely ubiquitous computing activity requires effective device and service discovery as well as appropriate media usable across two or more devices. In this paper we show how Session Initiation Protocol (SIP), the call signaling protocol for Voice over IP, effectively separates discovery from media-rendering selection in opportunistic augmentation. This separation improves system flexibility while allowing users or system administrators to choose the most appropriate discovery technologies for the environment. We also describe two phone-centric discovery mechanisms and demonstrate the practicality of the system by implementation and use in a test environment.