For a while now I've noticed calls coming in through my voice trunks with a strange caller ID.
That caller ID is +266696687.
At first I thought they might have been calls from an Australian 02 number with an incorrect conversion to E164 (eg: that number might have really been 02 6669 6687).
After doing some further digging I then noticed that the digits stand for ANONYMOUS as typed on a telephone keyboard.
I have come across a few sites indicating that these calls should be blocked however in my case they are legitimate calls from people with their caller ID withheld. As such if you're considering the same then I'd suggest proceeding with caution.