Ah, now, there's a thing. You see, I object to giving out my mobile phone number like that. It's the same reason I rarely give out my mobile number in general. I don't want the risk of random spam calls or texts, plus the fact my phone isn't always on - especially while I'm sitting here right next to my landline phone.

All fair points.
Is any system really going to be foolproof though?
If two-step authentication is really the way to go, then at least we should be presented with numerous options at sign-up so that we can make the best decision for us.
I wouldn't mind giving my number to Apple for this reason as long as there were assurances that no third-party would be given it and it wouldn't be shared with anyone else.
I intensely dislike the way some services require a secondary edress to send password recovery info, for instance.

I also personally dislike linking accounts together. My Twitter does not communicate with Fb, I very actively try NOT to sing in to services requiring me use Fb or Twitter.
I just find I instinctively distrust services that require other services' account details.

Mark

+1. Gmail and Hotmail continuously ask for mobile phone nos etc whenever I log in.

+2
But I never give them one.

Me neither. My number stays with me for the time being.

Mark