Payments on account, sole traders

Hello

I have had a call from a sole trader today very angry because they say I did not explain payments on account.  I filed their return and there was no tax to pay. I did mention there could be tax due next year and to put some money aside, ideally monthly.  They said they would have paid something now if they had known, I tried to explain how it works but they would not listen and now want to go to someone else.   Chances are this persons profit this tax year would not even generate much of a tax bill anyway, but there you go.

Anyone else had any problems with this sort of thing ??  I know it is sometimes a shock for people in first year of trading who have a tax liability and also have to make a payment on account!