Unexpected Charges and Terrible Customer Service from My Mobile Provider

I had a mobile contract for just over twelve pounds per month. During the contract period, I added another SIM card under the same agreement. Our plan included unlimited internet, unlimited calls, and unlimited messages. They also promised that if there was any unusual or excessive usage, they would contact me immediately so it could be stopped. Without any warning, they suddenly charged me over three hundred and fifty pounds. I called them many times, but nobody responded properly. In the end, I was forced to pay the full amount. Later, they told me that after making the payment, my SIM card would return to the previous contract terms. Now, when the bill arrives, it first shows a lower amount, but when I try to make the payment, another page appears saying I must pay nearly double. This company is dishonest and I would advise everyone to be very cautious before signing up with them.