Hello @Anonymous
Welcome to the forum and I'm sorry it's in these circumstances.
Did you send these items back via recorded or special delivery so you have return receipts ,and a signature.
I'm surprised O2 did not e mail you or send you a letter originally when the upgrade was ordered even if it was not by you ,or if it was me I would have refused the items when the courier delivered them to my home.
Whenever you speak to O2 either by phone or online live chat they should add notes to your account which would support your conversations.
Calls are recorded so if you spoke to them that way it's possible they are still available depending on time lapsed.
When you speak to online chat you have the option to keep the transcript or screen copy it at that time.
Have O2 sent you any letters asking for payment before the Debt Collection Agency got involved.
Either way id suggest putting all this in writing via ;
http://www.o2.co.uk/how-to-complain/complainAlso as this has gone to a debt collection agency I would also strongly suggest contacting a company such as Experian to see if a default has been added to your credit file, which even if paid would sit their for 6 years possibly affecting future credit requests. Unless O2 find it was them in error and contact the agency to remove the mark.
Let us know how you get on.
