For our PCM multiplexer, usually the longest distance from FXS device to telephone set is 4KM,and FXS--Loop Resistance: 1200 ohm. 

As it also related with the FXS line's material, so if user need longer distance, user can measure and calculate the total Impedance for the FXS line, or ask the FXS line's supplier, if it not exceeding 1200ohm, it is no problem even longer than 4KM. we have another type PCM multiplexer can support 1700ohm, if user need higher impedance for longer distance, please contact us.