New number for Tax Credit Helpline
The new Tax Credit Helpline number is 0345 300 3900.
You should only call this number if you have questions about your tax credits.
The exact cost of calling this number can be checked by calling your telephone service provider.
If you have hearing or speech impairments, the new textphone number is 0345 300 3909. You will still also be able to use the old numbers (with the 0845 code), for about the next 18 months.
Source: HMRC website