Lync Server Enterprise Voice uses the Exchange UM infrastructure to provide call answering, call notification, voice access (including voice mail), and auto-attendant services.
all of the notifications and voicemails are stored on the main mailbox of the user; which can have an impact on the user quota
so in order to roughly understand the impact "size on disk" for the voicemail; we have the below calculation.
The narrowband used codec is consuming 16Kbps; so the size on disk will be (16Kbps X 60 Seconds)= 960Kb / 8 bits = 120KB per 1 minute voicemail
so for a standard maximum 3 minutes voicemail message = 360KB
Also notifications can be turned off from the Exchange2010 OWA Phone>Voicemail settings under notifications menu.; You can disable save call logging on the mailbox from the client side or from client policy from the server;
this control can be done via PowerShell command (Set-CsClientPolicy –Identity Global –EnableCallLogAutoArchiving $false)