Unexpected high bill due to unauthorized SIM plan usage. What to do?

Dear Sir/Madam,

I need your help regarding my SIM Plan. When I signed up for the plan, I specifically requested to use my own number since it's with the same network. The agent agreed, and I provided my ID and signed the documents.

He informed me that I had to activate it before I could use it. However, when I attempted to activate it, it did not proceed, and I assumed that I didn't have a plan.

I did not contact the agent because I thought that since it was not activated, I would not be charged for anything.

After a few months had passed, I began to receive bills, and someone was paying for the plan. I then inquired as to why I had a plan when it wasn't activated. That's when I discovered that another person was using the plan under my name, and I had to pay to cancel it.

I don't know who that person is, but the agent informed me that I had to pay for it because it was under my name.

The amount reached over a thousand because the person did not pay.

Could you please assist me with this situation?

I am really confused and don't know what to do.