You might want to either add a feature where the display is lit at all times or an option to choose this in the menu. Doesn't matter how big the numbers are if the display goes dark ;-)

Dear Daniel, I paid almost one hundred euros last month because I need to limit my phone calls to 4 minutes and 50 seconds, but could not do it. Do you think you can develop a Timer that automatically A) start timer when phone call starts and B) terminates a phone call at 4'50''?? I will pay 50 euros for that application!