I'd urge caution on this. The world is not as simple as you think.
Some of the problems:
- People have multiple SIMs, and multiple devices. While this doesn't apply to everyone, a growing fraction of individual users will fall into this category. I'd expect far more people to have 2 SIMs than 2 nationalities/passports.
- A few people have no phone/SIM and no inclination to change. Some form of 'digital inclusion' policy would be needed for the cellular refuseniks.
- *EDIT* - also see my more recent post where I try to quantify the two factors above
- Many countries still have anonymous prepaid SIMs. While some countries like Italy & Thailand force SIM purchasers to register their ID, that's a far from uniform situation. I've bought loads of SIMs over the years with total anonymity. And I'll bet that black markets exist even in countries where registration is mandatory.
- Just because a SIM is registered to an individual, that doesn't mean they're the user. A speaker from India yesterday said that SIMs are usually registered to a head-of-household, rather than the actual user.
- No idea what happens with legal responsibility & identity with children with SIM cards, but I'll bet it's complex and variable per-country
- Plenty of SIMs are used by machines, not people. How do you know if it's actually a vending machine that's sending a message, or someone hooked up via a PC?
- Some phones or other devices are shared between multiple people (eg in 'pooled' devices in businesses, cellular fixed phones, or phones for a whole village in emerging markets)
- There is some evidence that young people swap phones (& SIMs) amongst each other in social groups. This could be because of the dynamics of who has prepay credit among friends, for example, or in situations where an operator doesn't offer x-network minutes and it makes sense to use someone else's phone to save money.
The last point is perhaps the most crucial. The whole idea of one SIM = one person it hugely exposed to the risk of social dynamics and fashions. It only needs one group of teenagers to come up with a 'killer app' for phone-sharing or swapping (maybe it becomes cool to have a new number every week), you get viral adoption around the world in a couple of months, and the whole identity/SIM linkage falls apart.
Ironically, the use of Internet access on mobile devices exacerbates this risk, as potentially users can sign into Yahoo/Google/Skype/Truphone/Fring from any device rather than their 'official' phone & SIM.