Received an SMS from this mob saying I owed $87.94...

Received an SMS from this mob saying I owed $87.94 to a company I had never heard of. Three phone calls later I was told it was a mistake and to ignore the SMS. I requested this in writing or at least as an SMS but was told that was not possible.
Why not?
What happens down the track when the matter is elevated to their lawyers and I have nothing other that a verbal statement from an unknown person that I owe nothing?
Not good enough.