The convergence of push-to-talk over cellular (PoC) applications like Zello with legacy analog telephony signaling techniques presents unique security and operational considerations. This paper investigates whether Zello’s audio channel can faithfully transmit a 2600 Hz tone — the historic “blue box” frequency used to seize trunk lines in the PSTN. Through empirical testing of Zello’s codec response (Opus, at varying bitrates), we evaluate the minimum bandwidth and latency required to avoid tone attenuation or harmonic distortion. Our findings indicate that with “best” settings — 32 kbps, mono, no noise suppression — Zello can pass a sufficiently pure 2600 Hz signal to trigger older in-band signaling equipment on vulnerable PBXes. We conclude with best practices for penetration testers and retro-telephony enthusiasts, and discuss mitigation for modern VoIP systems.
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Zello is a top-rated push-to-talk (PTT) walkie-talkie application used by over 150 million people worldwide for instant, live-voice communication. It turns your smartphone, tablet, or PC into a virtual two-way radio that works globally over any internet connection (Wi-Fi or cellular). Our findings indicate that with “best” settings —