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In this paper, we consider a university campus that has an established infrastructure for supporting SIP-based VoIP service through the campus wireless data network. The campus WLAN, however, does not have 100% full coverage, and hence users cannot make untethered VoIP calls anywhere on campus. The goal of this paper is to overcome the limitations of such WLAN dead spots and improve user experience when making VoIP calls. Different from related work that relies on peer-to-peer communication for multi-hop relay, we propose an approach called ldquodual-mode communicationrdquo to leverage the availability of dual-mode handsets for ubiquitous communication on campus. We implement the proposed approach and evaluate its performance in the campus testbed environment. We find that an opportunistic usage of dual communication modes on mobile handsets does allow ubiquitous voice communication in WLAN dead spots. However, it has one problem as the potential lack of voice call continuity during hand-off between the two modes that can cause degradation of the speech quality. We adopt a cross-layer solution based on signal processing algorithms to address the problem, thus achieving seamless voice call continuity while enabling ubiquitous voice communication on campus. Testbed evaluations show promising results for future research along the proposed direction.