Driving while using a mobile (cell phone) or other communication device is illegal, can result in a fine and penalty points against your licence. Even if you're stopped in traffic. You must be parked to use a phone in your vehicle.

Use a handsfree kit if needed and only push buttons while it's in the cradle.
http://www.roadsafetyni.gov.uk/index/faqs/faqs-mobilephones.htm

The automatic penalty is 3 points against your license and £100 fine.

(If you have a new license and you get 6 points in the first two years, you'll lose your license.) https://www.gov.uk/using-mobile-phones-when-driving-the-law