TELECOM Eireann got its lines crossed with 4,000 customers yesterday by not informing people that their number was up on April 1st.

Some people on Dublin's north side thought it was an April fool's prank when they found their telephone numbers had been changed. Although they knew their numbers would be changed, they did not realise it would actually happen yesterday.

And it caused chaos for businesses whose customers could not reach them because a free phone "help line" with a message detailing the new number developed a fault and was left, well, helpless yesterday morning.

To counteract the chaos, several companies have taken out advertisements in today's newspapers advising customers of the number change. They include Maxol, Readymix, Brooks Thomas, Burmah Castrol and The Stockyard Building in Upper Sheriff Street, which contains several buildings.

Some time ago Telecom wrote to certain customers informing them that they had found it necessary to divert their lines to other exchanges and that this would entail a change in telephone numbers.

Customers were told that the change of number would appear in the 1996/97 directory and the change of number would be effected thereafter. "We will advise you of the actual date of the change beforehand," it said.

One problem though Telecom is still doing the numbers and its new directory has not yet been published.

One business told The Irish Times that the change had caused severe disruption. "People here just could not be contacted," said a spokeswoman. "It caused havoc. People got a message saying consult your new directory and the directory isn't even published yet."

The company had to hurriedly fax out its new number to several clients.

A Telecom spokesman said last night that the free phone number which gives the new number had developed a fault, but it had been corrected by yesterday afternoon. He apologised for the fault.