Panasonic KX-TD500 System Reference Manual page 103

3) Maximum cabling distance of the extension line cord (twisted cable)
DI GI
TA
L SU
PE
R HY
BR
ID SY
ST
EM
RU
N OF
F LIN
E
AL AR
M
4) Auxiliary connection
Refer to the section 2-C-5.00 "Auxiliary Connection for Power Failure Transfer".
26 AWG: Under 2290 feet
24 AWG: Under 3700 feet
22 AWG: Under 5900 feet
2-C-25
Single Line Telephone