Are personal mobile phones now taxable?

It used to be if you had a mobile phone from the company you would be taxed on it as it was a "benefit" However if you used your own phone but expensed the monthly bill as it was for business it was all good. Im now told that it's the other way round. So if I use my own phone and pay £50 a month contract I have to pay £20 a month tax. If they give me a phone I pay nothing. I've looked at HMRC but its up to its usual standard of explanation.