Can’t pay my final bill

I bought a new phone not realising I was unable to keep my ID number. 
My old ID number has been discontinued and now switched off. I never received a final bill. I’m aware I have a small amount left to pay of my bill but unable to pay it. My new ID number has overridden my details on the app and online as I wasn’t able to add an additional number. When I log in online, I can’t see or access my old number anywhere. When I tried calling the number to pay the bill, because my old number has been switched off, it wasn’t able to connect the call due to the automated service. Please can someone help me with this? I do not want a bad credit rating because of this and desperately trying to pay my bill. This automated service is so frustrating!