This is the link to the Highway code: http://www.highwaycodeuk.co.uk/control-of-the-vehicle.html

Rule 123 states: “You MUST NOT leave a parked vehicle unattended with the engine running or leave a vehicle engine running unnecessarily while that vehicle is stationary on a public road. Generally, if the vehicle is stationary and is likely to remain so for more than a couple of minutes, you should apply the parking brake and switch off the engine to reduce emissions and noise pollution. However it is permissible to leave the engine running if the vehicle is stationary in traffic or for diagnosing faults.”

See also: The Road Traffic (Vehicle Emissions) (Fixed Penalty) (England) Regulations 2002
https://www.legislation.gov.uk/ukdsi/2002/0110423887

These are the rules on using a mobile phone:
https://www.gov.uk/using-mobile-phones-when-driving-the-law

You can use a phone only when “safely parked” which means handbrake applied and engine and lights switched off (Highway Code rule 239).

While on that topic, here’s some advice from RoSPA about distraction if using hands-free mobile phones:
https://www.rospa.com/road-safety/advice/drivers/distraction/