Whenever you make a telephone or email contact, it is essential to record your responses in a consistent manner - somewhere easily retrievable.
In the smaller business you can keep basic records through the file notes in your accounting packages. On other occasions, writing the story of your contacts on the back of a copy of the unpaid invoice is acceptable.
The purpose of keeping records is to
(i) ensure you know what happened for each contact;
(ii) provide a strategic picture of your actions so you can plan the next conversation or action; and
(iii) to support your case if you need to take legal action for the unpaid invoice(s).
Without such records, you are usually unable to control the next conversation and this puts you at a severe disadvantage.
One final hint; take notes in a manner that allows you to extract the applicable notes for any one account or invoice.
I know of one person who incorporated all their notes on a daily page of contact conversations. Therefore when the records were required for legal action of one account, the applicable notes needed to be found and then reformatted in to a single record of events for that account.