In the context of validation of numbers for ENUM, I have tried to
estimate the amount of mobile users which have totally anonymously bought
a prepaid SIM card, or in other words, users of E.164 numbers which have
no trace at all in a phonebook or other records.
In the EU, all countries except Germany, Hungary and Italy permit totally
anonymous purchase of prepaid SIM cards. This indicates that about 65% of
the EU population have access to anonymous mobile service (even in Italy,
just a taxpayer code is needed which is not subscriber data but allows to
track down a user for law enforcement - but lets leave that out for the
sake of argument; in France I understand a record is made for purposes of
recovering a lost SIM card but not for proof of identity).
There are about 400 Million mobile users in Europe, and penetration of
prepaid is about 40%. Assuming that nobody registers if he doesnt have
to, and multiplying those figures a ballpark estimate is that there
are about 100 Million E.164 numbers in use in Europe without any
subscriber data just through the use of prepaid SIM cards.