Many (older?) phone headsets use a 2.5mm 4-pole plug. Many airline AV systems use a 3.5mm TRS/Stereo/3-pole jack. If that's what you have, something like this will help:
That one's $10, some are as little as $5.
That said, decent earbuds with a 3.5mm stereo jack are like $15 (or less). These were recommended by Wirecutter as the best cheap ones:
Edit: May as well mention also. The 3.5mm TRS is a "mini stereo" plug. RCA is different - separate left (black or white) and right (red) plugs, often used on older receivers as a line in. See
https://geekmusician.com/rca-vs-3-5mm/