Analogue phone lines are limited in bandwidth therefore also somewhat iimited in possible adjustments. While such recordings can not and are not meant to be presented in full fidelity, there are possibilities of improving their comprehensibility especially for broadcast scenarios.
This artifact-full and heavily distorted recording is a result of both over modulation on the phone line itself (probably due to interoperability of digital and analog lines with differing codecs, transmission standards, ..) and the end-of-chain broadcast processing by an Omnia processor.
What tactics would you employ to soften the harsh "humming artifact"? It seems to span in both the 100 - 300 Hz and 1,4 - 1,7 kHz. A notch filter helps, but only improves the low end.