What I've learned in my short time here is first and foremost stay calm - difficult I know.
Write to them, don't phone, explaining your situation and stating that you will no longer deal with them by telephone. Send any correspondence recorded delivery (costs £1 for a standard size/weight letter). If they call you, tell them politely but firmly that you will only deal with them in writing then put the phone down. Don't get into a slanging match, no matter how tempting!
I'd make the initial letter polite but businesslike, and then if that doesn't stop them move immediately to the template letters here requesting a copy of the CCA and other documents.
If the debt is legit and you are struggling to make the payments then you should send them a budget sheet. Simply put, they cannot demand what you haven't got. If they took you to court and you showed a judge your budget, he'd only make you pay what you could genuinely afford. Plus it would go massively in your favour that you're making them a reasonable offer.
Hope that is of some use, I'm sure others will add something more useful