Me again 

Another one for all of you!!
ExH was always useless with money. Really really awful. Was in a lot of debt involved with debt collectors when we first got together and my parents and myself helped him massively improve his credit score to the point where we could get a mortgage.
Fast forward to now, I keep getting letters from a mobile phone company, just assumed they were circulars/adverts whatever so binned them. They kept coming so I opened one out of curiosity today (no visible name, hidden by envelope only address could be seen)
Turns out he's not been paying his bill. For a while - to the point where they've written and said his failed payments have been passed onto a debt collection agency. Letter doesn't state how much is owed. He moved out 18 months ago and I remember him getting a new phone/telling me about new contract March this year, so why has he given my address when he hasn't been living here for nearly a year??
So basically if debt collection agency come knocking at my door, what can I do? I have his new address but not sure what I can do now it's gone to the agency. Will they believe me when I tell them he doesn't live here and provide his new address?! Any advice appreciated! 