-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.