You are receiving spam calls which are generated automatically by dialling software dialling all numbers within a particular area code. Have a read through the VM 'Security Matters' and 'Home Phone' forums and this problem gets regular mentions most days. There are minimal features built into the VM landline service to protect against these calls.
In order to filter out the automated scam calls, you need to use a device which screens the calls and processes them before deciding whether to ring your phone or not. Some options are explained below for screening the calls