Rumliprucs

Hi there, I started a new job at the end of August and there was a 4 days overlap between my old job and new job. I only have 1 job. P45 was submitted to the new employer and on one part of HMRC app it does state that I finished my old job on the 2nd. All was fine till last month (January) when they put me on a BRX tax code (which means they believe I have 2 incomes - which is incorrect). I dug a bit around in my HMRC info and when I check my expected tax payment for next year it also calculates with my old job's income and taxes me in the next rate bracket on my current job. I can't reach a human being on the phone at HMRC and wondering what can be done and what has not been processed? Thanks