There are other ways of contacting Eon Next (besides private message to Hannah on this forum) namely email or physical address; either way you have proof; the latter being recorded/special delivery. It is important you pay (even manually, if under protest) because in my experience, I was harassed by a debt collection agency, after an outstanding bill was paid manually with another energy company (I refrain from naming them on this board), after I questioned it.
Best course of action is to log an official complaint in the first instance; this will cover you in such an eventuality.

Eon Next Customer