The Revenue Commissioners have urged people who cannot get through to the Local Property Tax helpline to leave a message and it will be recognised that they made an effort to comply with the filing deadline.
The deadline has been extended until 5pm tomorrow following an "exceptional level" of filing over the last two days and an unprecedented volume of calls to the helpline.
Revenue has received 70,000 calls on the property tax since Monday.
It has now increased the number of operators taking calls to over 300.
Local Property Tax Manager Vivienne Dempsey has urged people to be persistent when attempting to call the helpline.
She said if they could not get through, callers should leave a message with their name, PPS number and a contact number so that Revenue can return the call.
Ms Dempsey said people should be reassured that if they leave a message with their details it will be recognised that they have made a genuine effort to comply, and this would be taken into account with regard to the deadline.
Revenue has received 870,000 Local Property Tax returns to date, with over 250,000 of those being received since Monday.