To those wondering where they're getting your number, it's fairly simple, your oh-so-angelic mobile telco is selling your data whether you like it or not.
I use T-Mobile* and I'm getting about a call per week from a company saying "we're calling on behalf of T-Mobile as your contract is coming up for renewal". When you push them they always admit they're third-party companies who have bought my details so that they can resell me a new contract and take their cut.
T-Mobile had a warning about this two months ago when they promised they'd never given my details to any third party. I re-confirmed that I originally signed, and still am, on their no-marketing and no-third party lists. Since I'm still getting spam calls, I've sent them a data protection letter telling them they've been naughty and copied in the Information Commissioner.
*brain-fart of a day when I signed up with them... I don't care who I go with when my contract expires as long as it's not T-Mobile.