Seems you have the similar issue, opening the Phone Link app just hangs on the first screen saying "hold on while we get your device ready" and spinning endless here.
Also, it may say something different but still you can not continue and start using it, i think these steps will help you solving this issue.
First step - go to Settings > Accounts > Email & accounts
Second step - scroll down to the "Accounts used by other apps" section and find there you microsoft account
Third step - change its "Sign in options" from "Microsoft apps can sign me in" to "Apps need to ask me to use this account"
Now try to open the Phone Link app. It should start asking you about your microsoft account and continue through whole process.
Happy Phone Link use! ))))