Spark customers have been affected by a nationwide system issue this morning that has seen 20 per cent of calls unable to connect.

The system failure has been caused by a breakdown in the VoLTE (Voice over LTE) system.

A Spark spokeswoman said the issue was nationwide but did not affect specific customers or areas but affected 20 per cent of calls.

The telco had a large team of people working hard to resolve the issue.

Customers using the 3G network were not affected.

"We apologise to any customers who are impacted and our team are working hard to resolve the issue as soon as possible.

"Landlines and calls to 111 emergency services are unaffected."