Need advice about debt collectors and vodafone!?

I havent paid a very large bill i got of vodafone, in fairness vodafone have only sent me about 2 letters about the bill and iv sent letters to them to try and sort out why my bill was so high, i dont agree with the bill myself and wanted questions answewrd. and in the last 2 months i havent heard ne thin from them. and then all of a sudden i get 2 letters from debt collectors demanding it. I dont have the money! what can i do? im a student!