skype app password
Update Skype for business to use regular authentication and multi factor (instead of app password when MFA is enabled)
Kevin Stecyk commented
I just spent over an hour with support because I didn't realize that Skype for Business required an app password. I have Multi-Factor Authentication and Mobile Device Management. Neither I nor the support person realized that I was using the wrong password. Everything else on my iPhone and iPad uses my Office 365 password.