Thank you, will my partner have to be with me when i make the phone call (when i pluck up the courage?)

me and my partner moved in together in July 2014 and i have forgotten to let tax credits know to change to a joint claim (Only realised today) my partner works full time and brings home around 1400 a month. Were in loads of debt as it is My anxiety is kicking in overdrive thinking im going to get fined and sent to prison! I havent phoned tax credits yet as im petrafied. any advice would be great