Ragnor: The fastest and least annoying way to report these kind of issues to Telecom so they know about it and can fix it is via Email us form on
http://www.telecom.co.nz/help/other/contact
The problem, especially from my POV, in reporting this problem is that is infrequent enough to be hard to reproduce, but frequent enough to be annoying. And as a software developer, I fully appreciate how difficult it can be to fix an issue you cannot reproduce. Or to know with 100% certainty that you've actually fixed it.