You're probably thinking of robbed bit signaling: https://en.wikipedia.org/wiki/Robbed_bit_signaling

This wasn't quite doubling. Five out of six frames were sampled at full 8-bit resolution, while the remaining frame was sampled at 7-bit resolution (hence the "robbed bit"). The "doubling" is probably that you could run 24 digital lines with the 7/8-bit encoding, over the wires used by just two analog phone lines.