Expenses claim help

Hi, I receive travel expenses from my employer at 25p per mile, I submitted a claim to HMRC for the other 20p per mile but they came back saying I owe them and ended up with a tax bill. I have tried numerous phone calls but as I never speak to the same person I never get this resolved, can anyone offer advice please?

Hi
We would not be able to confirm the reason for this without checking your record.
You will need to contact us again.
Income Tax: general enquiries
Thankyou.
Regards.