Spark customers were affected by a nationwide system issue this morning that meant 20 per cent of calls were unable to connect.
The system failure was caused by a breakdown in the VoLTE (Voice over LTE) system, but had now been resolved, a Spark spokeswoman said at 5pm.
The issue was nationwide but did not affect specific customers or areas but affected 20 per cent of calls, she said.
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.
