Dialogic 6.2 - Manual (Page 201)

Making Two-Channel Call Transfers (Tromboning)
November 2009 201
If the application configured the echo canceller in the channel to get
an input reference signal from slot #1, then the application must
remove this configuration to return the echo canceller to its default
behavior. To do this the application must call the
BfvSpeechEchoCancelControl() function (See Volume 3 of the
Bfv API Reference Manual). For example:
BT_ZERO(speech_args);
speech_args.echoc_op = ECHOC_OP_ALT_INPUT_DISABLE;
BfvSpeechEchoCancelControl (lp, &speech_args);
Calling the BfvLineReset() function on a channel also returns the
echo canceller to its default behavior.