http://www.theregister.co.uk/2017/12/19/dump_your_mobile_provider_by_text_by_2019/
Under the incoming rules, a user who wants to switch can now send one of two codes (which will be identical across all providers) to their current service provider, either by free text, email, or phone call. One code simply cancels the contract, the other requests the PAC number.
The provider then sends a cancellation code, which remains valid for 30 days, along with important account details to the user. The user can then send the code on to their new chosen service provider, who will set them up within one working day, with no notice period charges or other fees to pay.
Ofcom reckons this change will save UK customers around £10m a year.
To give service providers a chance to update their systems to deal with these rules, Ofcom's deadline for compliance is 1 July 2019.
