Re: And landlines !
Record the 'di dah dee' number disconnected tone on your voicemail. This causes a lot of diallers to hang up. Your friends and family will get used to waiting through the beeps and you can just pick it up later.
I have a phone with a VIP feature, where someone can enter a code and it cuts out of the voicemail back to the phone. The normal ring is set to zero volume, and the VIP ring is as normal. The message says that if you are a friend, colleague or relative, you can enter the VIP code. If you don't know it, you can hold on and this message will tell you. However, using the code if you are not a colleague, friend or relative is an offence under the Computer Misuse Act ...
Most uninvited callers hang up at that point.
Then it tells the caller what the code is. I noticed that some callers or machines are resolutely determined to hold on till the call is answered or a voicemail beep is heard. So I play them several more minutes of music, and then sorry, sorry voicemail is full, and the line is dropped.