The maximum distance is established by a total allowable loop resistance, including the phone or terminal equipment, of 600 ohms. Cisco FXS interface meets Operations, Process & Systems (OPS) Class B specifications. Specifically, it can source greater than 20ma to a resistive distance of up to 1100 ohms. The homework question is translating impedance to actual distance in meters in a production environment; which depends on the gauge of the wire and how the wire is routed and punched-down. It depends on the type of phone and equipment hooked on the line, as well as to how well that equipment works.
The formula to calculate DC loop resistance is:
LRi = CRi * SLi
LRi represents the DC loop resistance for the cable section.
CRi represents the Conductor resistance per unit length for the cable section. This is used to gauge the 24 rj11 cable.