-Only for Brazil-
There are some telcos which do not support
blocking collect calls via group B tones (Reject incoming collect calls feature),
instead they rely on a double answer mechanism where the answer signal is
sent and a few ms later clear back (hangup) is sent, then a few ms later
answer signal again, the telco (or PBX) will hangup the call with the first
clear back if it is a collect call, or ignore that pulse if is not and just
proceed to answer.