Microsoft planned to begin upgrading handsets to Windows 10 Mobile in December, but last month, it said that the update would actually arrive "early next year".
But the company has said nothing about its plans since then, and there's been considerable confusion about the rollout, given that some carriers have already announced that the update is ready to roll - and there have even been (unverified) reports that some users' handsets have been upgraded already.
The confusion has deepened in recent days as one carrier - which had previously said that its handsets would be upgraded this week - suddenly declared on Wednesday that Windows 10 Mobile won't begin rolling out until late February. Curiously, the tweets in which it revealed that information have since been deleted (although you can still see the content of those tweets, and find out more about that saga in this article) - but today, VentureBeat reports that Microsoft has written to its partners to confirm that the rollout has actually been delayed until next month.
According to that report, Microsoft said in an email to its mobile partners that the update isn't yet ready for release, and won't be made available until 'early to mid February'. The email was also said to reveal that the monthly update for the Lumia 950 and Lumia 950 XL planned for January has been pushed back until February too.