In NZ you sign up with an ISP, but your local connection is usually handled by the same physical equipment (DSLAM for ADSL, etc) which is owned by a single network provider.

I’m not sure what the incentives are for an ISP to try to get the provider to fix issues, or even if they would e.g. https://company.chorus.co.nz/what-we-do is notoriously bad for service and the copper network is being deprecated. Locally https://www.enable.net.nz/about-enable/ are doing a good job of service, because they are well subsidised by the government and seem to be effectively operated.