Hi B74,
At HMRC we do not have a contact number for DWP in Northern Ireland for queries regarding missing credits.
There is a NI Direct Government Services website which provides useful information for customers living in Northern Ireland:
https://www.nidirect.gov.uk/
However, the correct action for you to take regarding your missing Working Tax Credit National Insurance credits, would be to contact HMRC on 0300 200 3500 and we will investigate this for you. Lines are open from 8am to 6pm, Monday to Friday.
Thank you.