Rule of thumb when dealing with echo:-
Echo on one end is typically generated at other end
Echo is not introduced on digital links
Introduced by 2 to 4 wire conversion in hybrid and impedance mismatch
-8dB to -16dB drop end to end (-12dB being nominal)
Know loss plan of the PBX (AGC, Padding)
# or * DTMF tones approximate 1004Hz @ 0dB tones