I'm being harrassed by Virgin Active Debt Collection Agency, MDB

I cancelled my Virgin Active contract due to health reasons. I sent several emails to Virgin Active explaining this and was in contact with their Customer Service Rep trying to resolve the issue. After a while she stopped responding to my emails - I sent several emails to her and her colleagues and didn't get any response.

Instead I'm now being harassed by their Debt Collection Agents, who are extremely rude and have no background to my issue.

We didn't even agree how much I owe Virgin Active... if at all. But according to MDB I must pay.

My question is pay what???