The term "free minutes" is actually quite deceptive.
In reality, free minutes refers to the amount of contracted minutes you're allowed to use based on your phone contract and amount you pay each month. The only time minutes could really be considered free is once you've hit the contract threshold of having "unlimited minutes".
On the other hand, if I pay £40 per month for my phone contract, I may be eligible for unlimited minutes of call time without being charged any extra. In this instance, you could consider that once I've used enough minutes to cover the £40 contract payment, any minutes I use after that are free.
Free minutes websites
There are a number of websites on the internet that claim to offer ways to hack your phone to be provided with free minutes. I'd advise that you stay away from these sites as they're unlikely to work. Even if they do work, you're breaking the law and will likely be caught and prosecuted.
Ultimately, the only way to obtain free minutes on your phone is to pay for a contract that offers unlimited minutes. This is only worthwhile, however, if you use enough minutes to warrant not paying for a cheaper contract.