It was a peculiarity of the 2G signal. While the frequency of the signal itself was high enough that it did not interfere with speakers, the channel was shared

It was a peculiarity of the 2G signal. While the frequency of the signal itself was high enough that it did not interfere with speakers, the channel was shared between phones using TDMA, in which the channel is divided in fixed-size slots, and a phone transmits in only one of these slots while remaining silent for the rest of them. It's the envelope of these transmissions (transmit for a while, stay silent for a while, repeat) which caused the interference with the speakers. According to Wikipedia (if I understood it correctly), the interval between the start of one transmission and the next one is 4.615 ms, or 9.230 ms for half-rate, which corresponds to 217 Hz or 108 Hz (if I did the calculations right), well within audible range. Newer cellular standards use CDMA or OFDMA instead of TDMA, so they do not have that same low-frequency on-off behavior.