Not sure why they are doing this, and they seem to have it quite well
hidden, but I found it and it works.
So grab it while you can.
http://www.o2.co.uk/termsconditions/currentpromotions
Number One terms & conditions
The Number One promotion is open to new and existing O2 Online Pay &
Go customers who opt into the promotion between March 29th up to and
including September 30th 2007 (“the Promotional Period”) subject to
the terms and conditions below.
Eligible customers who opt in to Number One during the Promotional
Period will be entitled to up to 500 minutes/month of free calls to
their one nominated O2 UK mobile number for up to 3 months from the
date that they receive a confirmation text message from O2, subject to
a minimum top up of £10 in the month prior to the date of opt in and
each subsequent month during the 3 month promotional period.
Customers may change their nominated number once every 30 days by
texting CHANGENUMBERONE followed by the new O2 mobile number they want
to change to, to 21300 free.
To opt in customers must send a free text message with the keyword ONE
followed by their nominated number to 21300.
Customers will receive a text message from O2 confirming when they
have successfully been opted in to the promotion.
Customers who are currently receiving the O2 Long Weekend free texts
promotion (available for opt in between 5th January 2007 and 31st
March 2007 and lasting for up to 3 months subject to £15/month top-up)
can only opt into this Promotion once their current O2 Long Weekends
promotion has expired.
Existing customers on the Free Best Friend promotion currently
receiving free calls to one UK Mobile number are not able to benefit
from this promotion.
Eligible customers who do not top up by a minimum of £10 in their
first and second month will not receive the Number One benefit in
their second and third month. A reminder text message will be sent 48
hours prior to the end of your first month and/or second month if you
need to top up.
Free calls made as part of the customer’s Number One allowance will
not count towards the first three minutes of the day rates for
customers on Pay & Go Talkalot
Any calls made in excess of your Number One Allowance will be charged
at your standard rates. When your allowance runs out you will receive
a text message within 4 minutes to notify you that your free allowance
has run out and you are now being charged for all calls.
Where a customer already has a Bolt On or free allowances such as Talk
Anytime that includes voice calls, then the Number One Allowance will
be decremented first for eligible calls.
Unused Number One allowances cannot be rolled over from one month to
the next.
O2 reserves the right to amend this promotion at any time. Any
changes will be posted on the O2 website at
www.o2.co.uk
Customers may only opt in to this promotion once. If customers opt out
of Number One and then opt in again within the Promotional Period the
request will not be accepted.
Customers who disconnect and reconnect for whatever reason during the
Promotional Period will not be eligible to opt in to this promotion
for a second time.
After expiry of the three month offer, standard call charges will
apply.
02 promo - Number One - = 500 FREE anytime minutes to your designated number for the next three months 
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