OK I have had it with cold callers. We take about 5 calls a day from unwanted callers. Yes our office manager acts as a gatekeeper but this is still very inefficient and some do get through.
TrueCall is a gadget that goes between the master socket and the phone. It works well with cordless phones with base station. It is not cheap but it allows us to create a white list for clients who get straight through but "zap" people selling loft insulation, etc (How satisfying). You can even terminate the call mid-flow with a polite message. If they call back they will be blocked!
I have tagged this article "Compliance" because another feature is the ability to record calls (up to 140 hours). This allows further data to be gathered and added to the client's Evernote file. There are some legal implications of how to do this but this is not the place. A given phone call can be set to record mid-way through the conversation and the systems "buffer" will record the start of the call too.
The unit "phones home" once a week to upload call lists to the internet so that activity can be tracked. It also acts as an answer phone. It was featured on Dragons' Den (and highly rated)- but it does highlight that they buy the things in for £30 and retail for £100+. So there is your downside!