Distance is calculated based on REN numbers. REN stands for Ring Equivalence Number. This is usually listed on the phone or device you attach to the voice port. The max you can support on each port is 5 RENs. REN is a measure of the the AC signal that is used to generate the ring tone. So if you have one device connected, I believe you can go upto a kilometer. Default value of REN on a FXS port is 1, though you could configure it to 3 or 5. A show voice-port x/y can show you the REN setting (check last line).
Hope that helps!
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus
