But you know ahead of time whether it's opt out or opt in, and you make arrangements accordingly.
I don't want my mobile number shared. I've never been stalked and I don't think of myself as being at high risk of being stalked. But I always always always tick the 'no contact' boxes for email and whatever, and even with that, my spam folder is full every day. I don't give my mobile number out unless I must and I still get spam calls. My home phone is unlisted and I still get 'do I want a new boiler' calls, and 'mis-sold pension plan' calls and 'have I ever had debt insurance' calls. Sure, if I volunteer for the tea stall at the local branch fair for the Dinosaur Defence League, the five other women manning the stall may need my number, plus the three overall co-ordinators. Seven thousand national volunteers who are running the regional fairs for all the other branches don't need it and in my opinion have no right to have it.
I'm not afraid that they'll stalk me. I'm afraid they'll sell my details and for the rest of my natural life I'll be swamped in junk mail. You like Dinosaurs! Now rescue a unicorn! You made chocolate cake for them! Make it for us! You can afford to do charity work! You need a financial adviser!
My SMIL went looking for a repairer for a household appliance; they did the repair, did a service, and asked her if she knew anybody else with the same sort of appliance. She did: me. She gave them my phone number. They currently call me once a fortnight asking if I want the service done; I don't. I can't block phone numbers on my landline and I run a business from home so I can't not pick up calls I don't recognise. I am extremely annoyed that she gave out my number in the first place.
And you know, on here, most of us aren't using our real names. (At least, I hope we're not, or the Bad Baby Name websites aren't showing the half of it.) We say that we live in this area or that area, but we don't say 'I'm Susie Cricket and I live at 49 Glamorgan Avenue, Colchester CO1 1AA'. We don't offer our phone numbers to strangers on here. So maybe there are one or two people I'm on personal terms with, and we do share details. Would that make it acceptable for somebody to post my address here, just because she knew it and I hadn't expressly said not to?
You don't need to be afraid of stalkers not to want your number given out at random. You might be, say, a politician. A police officer. A tax inspector. Criminal lawyer. Somebody who makes the final funding decisions for social security cases. Somebody who works for an animal testing lab. Somebody who works for the genetic engineering companies. Somebody who works for a hunt - or somebody who works for an anti-hunting charity. Somebody who works for an armament supplier. Soldier, sailor, RAF officer. Provider of contraceptive care. Social worker in charge of child abuse cases. All pefectly legal, and all good reasons to keep your number circulating only where you know about it.
I've never been a big believer in 'if you've nothing to hide, you've nothing to fear'.