Nexiwave offers serveral overage purchase plans. You can choose the pricing plan in your Nexiwave's Subscription page (see "Current Plan" section -> Overage Purchases).
You can easily turn it off in your Nexiwave's Subscription page (see "Current Plan" section -> Overage Purchases).
When overage purchase is turned off, Nexiwave will still send voicemail emails to you, except the transcription field just contains "Voicemail transcription limit reached ...", with the audio file attached. You can still listen the attached audio file for the voicemail.