I, too, worked in telecoms (BT, 1973 to 2005). I was told that voltage spikes on the line was one cause of the clumping. The trimphone - which had a different, and much smaller carbon microphone (sorry, "transmitter") - was very, very prone to microphone crackle. The official fix was to fit one of those bi-directional voltage clippers across the microphone, thus protecting the microphone from the voltage spikes.
I can't remember now how successful it was - the trimphone was never that popular anyway.
What technology did those voltage clippers use? Back-to-back zeners? Or something else? We used to call them "smarties".