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To be fair DECT phones broadcast around 10-50mW and a cable modem can transmit at up to 13mW so a hundred of them together would be far in excess of a DECT phone rather than not even remotely close. However that said the cables are shielded to keep the signal in, rather than designed to radiate it out as effectively as possible so the actual RF output is far far lower.

There may be more leakage coming from the 100v signals used in the phone lines and some people are far more sensitive to others so I wouldn't discount it having an effect.