When the operator transfers an active call, there is no ringtone, just silence until the party picks up. If the party doesn't pick up, the user hears only silence and doesn't know what is going on.
FOP2 does not produce nor is responsible of audio / sounds. It is asterisk and your configuration that does that. It is not a FOP2 issue but a particular configuration of your setup. Not sure what could it be as it does not happen normally in any setup I know of.
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Best regards,