EE, formerly called Everything Everywhere, is the largest mobile phone company in the UK. It is particularly well for its 4G coverage. They even provide coverage on the London Underground. Although previously owned by Deutsche Telekom and Orange, it has been owned by BT since 2016.
01 numbers represent typically represent geographical numbers in the UK. They are priced between 3p and 55p (approximately) from the provider EE. It is always a good idea to contact your mobile phone provider if you require an exact price. This only applies to the UK, phoning from abroad may cost even more. The 01 number range is available for purchase here.
It is recommended that you contact Ofcom if you have any problems with your phone bill or any specific charges associated with your mobile phone provider.