OMTP Legacy Headset Standard

Open Mobile Terminal Platform standard, predating CTIA’s AHT by 5 years. Specifies 3.5 mm TRRS pin assignment with Microphone on ring 2 and Ground on sleeve — the inverse of CTIA’s later standard.

Used by Nokia, early Sony Ericsson, Samsung pre-2012, and most pre-Apple-iPhone Android phones. Effectively obsolete in 2025 — no modern phones ship with OMTP wiring, and the legacy headsets that used it have largely retired.

Survives only in the headset auto-detection logic of modern phones, which interrogates the connected accessory to determine CTIA or OMTP wiring before routing audio signals. See CTIA AHT for the modern equivalent.