First thoughts were this:
http://forums.moneysavingexpert.com/....php?t=4858898
If you've been assured the only additional cost is £3pm on your existing contract then there's plenty of theories, I'm sure someone has seen this exact one somewhere before.
- Plan to keep you as a customer when your current contract runs out
- Plan to tie you into a new contract.
This one appears to fit the bill a bit more:
https://forum.vodafone.co.uk/t5/Pay-...t/td-p/2433255