Around 20-30% of the incoming voicemails are actually empty - and of course we would like to remove this as a burden of administration by dropping or auto-closing these emails.

I propose an option in the spam filter for attachment size (which would be used in combination with the sender/subject)

Alternatively a field you can use in business rules of 'Total attachments size' (given that it can apply to all tickets, and it would be hard to list details of one attachment)