Telecom is apologising to some South Island customers who are having difficulty with phone calls.
There have been a number of reports of garbled voices during mobile phone calls and the source of the fault is not yet known.
A Telecom spokesman says the fault is only affecting those north of Timaru, but it excludes those in Christchurch.
He says a team of 10 is working on identifying and fixing the error.
The issue started last week but extreme weather in parts of the South Island has complicated testing.
