[noobermin]

Generally datasette formats were meant to work with the extremely lossy media of tape. Also, I'm not sure but telephone audio tends to be much more compressed compared to radio. That on top of the fact you are going from earpiece to mic with an air gap and the background noise, it's probably much much worse.

[ale42]

Regular telephone audio is usually filtered 0.3-3.4 kHz (this on analog phone lines). Digital phone lines use PCM with A-law (or µ-law in the US and some other places) logarithmic sample encoding, with 8 kHz sampling rate, getting more or less the same result of an analog phone line, possibly with less distortion. FM radio has much higher bandwidth, usually up to around 15 kHz and with better basses too.