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Jackal24 wrote:

Any ideas on building a cellphone adapter for aviation headsets?

IMHO simplest & chepest solution is "unmount" cellphone headset with switch, and
stick it "into" regular aviation headset - mic on the arm and small speaker inside
headphone. Cellular activate-deactivate switch somewhere fixed outside mic arm.

Disadvantage is second cable with connector.

Regards, Mik