Spark is asking people to use internet apps to contact each other as overloading continues to plague the traditional phone systems.
The telco updated on its website and social media late this afternoon: "Customers may be experiencing a loss of service for voice calling.
The New Zealand telecommunications industry is working quickly and collaboratively to remedy calling issues that have stemmed from a significant increase in voice volumes on both mobiles and landlines.
New Zealanders are encouraged to use internet-based calling options such as Skype or Facetime until this issue is fixed."
Vodafone has been the first telco to respond, with a spokeswoman telling the Herald, "Voice call volumes are up approximately 20 per cent as more Kiwis are working remotely.
"Today's issue is impacting the full industry and we're fixing it as a matter of priority, which includes working with other telcos on call interconnects.
"There has also been a spike in suspected spam calls from overseas numbers, which we're working to block."
Geoff Thorn, head of the Telecommunications Forum, whose members include Spark, Vodafone, 2degrees and Chorus, said, "The New Zealand telecommunications industry is working quickly and collaboratively to remedy calling issues that have stemmed from a significant increase in the volume of voice calls on both mobiles and landlines.
Mobile operators have been experiencing high call volumes which is resulting in some points of congestion across the country, Thorn said.
In the meantime, anecdotal evidence suggests some networks are gumming up as many white-collar staff work remotely for the first time - and, it seems, more are picking up the phone than usual as they seek to keep in close contact with fellow staff or clients.
After this reporter tweeted about not being able to reach a landline in Whanganui, many got in touch saying they had experienced the same problem with attempted calls on, variously, Spark, Vodafone, 2degrees and Spark-owned Skinny.
"Trying to call a landline from my mobile and repeatedly getting a 'Your call cannot proceed at this time due to congestion.' ... People trying to call me not connecting. I'm with @vodafoneNZ," said NZTE marketing manager Amanda Pausina.
Enterprise architect Mark Heseltine said, "Same here, on Skinny mobile, trying call to @2degreesmobile."
And Mike said, "Seems the 2degrees network is having major issues.. you can call other 2degrees numbers but when anyone calls them or they call - they get a 'you cannot be connected'."
Meanwhile, broadband appears to be holding up okay.
Chorus said Friday was the first day it had seen a "noticeable" increase in daytime bandwidth.
But the network operator also said it had the capacity to handle three times the traffic it registered on that day.
