Letter from o2 got 20 days to contact them, no money, no job, what to do?

Hey, after losing my job, I missed payments on my mobile phone contract, it got disconnected and I owe o2 money. They wrote me a letter asking me to pay up the money I owe and gave me 20 days to get in touch. The problem is I have no money to pay them, no job and am claiming no benefits so have no income at all just now (I live with parents so they pay for food etc. but will not lend me any money, and I wouldn't want to borrow from them). I am actively looking for a job though but have no idea when I will get one. What should I say in a letter to o2? will they understand if I say I can't pay anything just now until I get a job? What's my best plan of action?