outgoing.go
func (ov *OutgoingVoice) SetDuration(to int) *OutgoingVoice {
	ov.Duration = to
	return ov
}
func (ov *OutgoingVoice) SetDuration(to int) *OutgoingVoice {
	ov.Duration = to
	return ov
}
// SetDuration sets a duration of the voice note (optional).