HMRC: Online Services helpdesks

You can also call this number if you need information about the known facts required to enrol for the HM Revenue and Customs (HMRC) online services listed above. Opening hours: Monday to Friday, 8am to 6pm Closed weekends and bank holidays 0300 200 3701. For customers who are deaf or hearing or speech impaired: 0300 200 3719 (Textphone).

https://public-online.hmrc.gov.uk/information/helpdesk