Bad Service and Follow-Up

I recently paid off my debt that had been paid in full on 10 November 2023. I phoned them on Monday 13 November and was told I am to respond to a sms link I would receive which took me to an email address *** stating I require a paid-up letter. I tried to follow up with them numerous times by email but there is just no response. I called them and have managed to chat to them twice since but most of the time the line just cuts off or it just rings and no-one answers the phone. The last time I spoke to someone I was told it takes 14 working days to receive the letter. It is now 18 days and still no letter received. I am concerned that there is no sense of urgency from them regarding this and that I do not get any response whatsoever on email. Bad service overall.