Office 2021 phone activation isn't working

Microsoft has been moving phone activation away from the old automated phone line, and on a lot of installs the activation wizard now points you to a web page to enter the installation ID instead. So if you're dialling a number that just hangs up or loops, look again at the wizard's screen for that option. Also worth checking where the key came from. If it was a cheap one from a marketplace, those are often keys already used on lots of PCs, and activation will keep failing whichever method you try. A key that came with your Microsoft account (bought from Microsoft or a proper retailer) can just be signed into instead of activated by phone.