That's not correct.  The Intuit accounts your clients use to log into Intuit Link belong to and can only be managed by your clients.  There is nothing, whatsoever, you can do to setup, manage, or delete their personal accounts.  What PS allows you to do is to "connect" your Intuit Link account to your clients' through this invitation process so that you can use Intuit Link as a common portal between you and your clients for a particular tax year.  This is how Intuit's ecosystem works.

The phone number you update in PS is only your internal record and does not change anything in your client's Intuit account.

Your client should have an option when logging into Intuit Link to indicate that he has no access to his phone so that an email will be sent to the email address associated with that account.

To update any personal particulars, including option for 2FA, your client must log into One Intuit Account (OIA) at https://accounts.intuit.com to make those changes himself.