The changes in Northern Ireland telephone numbers could have an adverse effect on small and medium-size businesses involved in cross-Border trade, according to the Confederation of Small Businesses in Belfast.
Since the introduction of new codes on Saturday, Northern Ireland companies are reported to have lost lucrative contracts because both their customers and suppliers in the Republic were unable to get in touch with them.
A spokesman for the confederation, Mr Bill Jeffrey, said once all companies had returned from their Easter break the full extent of the problem would be revealed.
"I am confidently predicting it will be a shambles. There will be frustration and anger when customers and suppliers find out that they are unable to get through."
He said there was an urgent need for cross-Border co-operation to clarify the situation.
The confederation said it was aware of one particular company which had lost a £19,000 sterling contract when one of its clients from the Republic pulled out due to communication problems.