Under the old State Pension rules, up to 5 April 2016, you were able to ‘contract out’ of the additional State Pension. This meant that you and your employer could pay less NI contributions into the state system. You could not contract out of the basic State Pension. You could only opt out (‘contract out’) of the additional State Pension, and you could only pay less NI contributions into the state system if you were part of a private pension – such as a workplace or personal pension scheme – that could build up to replace the State Pension you were opting out of.
More info - https://www.gov.uk/contracted-out