Some people can expect a $20 shock in their next phone bill.

Chorus is changing the way it charges line rental on Monday.

Spark and Vodafone have chosen not to pass on savings to 20,000 landline customers who have their broadband with a different provider.

TUANZ CEO Greg Young says customers have several options.

"Perhaps going to another service provider and bundling with another service provider, cancelling their phone line, perhaps going to naked broadband, or even perhaps just ringing Spark and complaining."

He says it'll affect tens of thousands of people who have their landline with Spark or Vodafone, and their broadband with a different provider.

"Spark's underlying cost for that phone line will be going down and Spark and Vodafone as well have decided they are unable, or unwilling, to pass on that cost reduction."

Mr Young says those people will see their bills increase by $23.52 a month.

Prices at other providers will remain the same.