struggling to pay an outstanding account

I bought a new phone as an upgrade around 12 months ago and forgot to transfer my old plan. I set a new DD on the new plan and rather stupidly  just put in my previous plan SiM to keep the number. I discarded the new plan SIM card. I tried to sort the 2 plans I have out a couple of months ago and stopped the DD on the new plan. The plan was reinstated after a long AI session. However I have an outstanding payment to make.  I I have tried all ways to pay it but I don’t have the new plan added to my account. I can’t set up the new plan because ID want to send a set up code to register the plan to my new plan SIM which I don’t have - I do know the  phone Number and account but can’t get a way to pay this and have had a final demand letter. Bring back a live person as this online stuff is totally frustrating.