Emails saying owe money

I left Bulb 2 years ago. In December 2021 I started getting emails stating I still owed £270 pounds and to “click here” to pay. When I did that, it took me to a page that stated my final bill had been settled and there was nothing to pay. I presumed then this that it must be a mistake or a scam to ignored the next few emails. I then started getting emails, and a letter stating I needed to pay or would be sent to a debt collector. I now have had a final reminder and have been told I will be sent to a debt collector on 10th April. This is very concerning and upsetting. I have tried to telephone Bulb. They are only open 9-5 Monday to Friday so it is difficult to get time to do this as I work full time. Today I was on hold 30mins and still no answer. I have tried their chat function on their website, but it always says there is no one available to help me. I have replied to the emails and sent an email but nothing. What I am supposed to do , I do not want a mark on credit file .