This is not only a telephone recorder symbol and the history of this particular symbol should be seen in radio receivers. So back in the day when anything with speaker was expensive as hell, a typical "upgrade path" of a tube radio was to buy a vinyl disc player. This player came without amplifier and was connected to the radio using a "banana" jacks or a "DIN" connector. The icon, reminding a vinyl player, was used then not only in connectors, but also in switches: AM, FM, Middle, Phono - the last one. Later it was used in radios as a general purpose input, without a frequency filter needed for vinyl players. It sometimes was used in recorders too, or, generally, as I/O DIN connector for radios (I/O - when radio was receiving it was used to connect tape recorder to record the transmission).