Office365 password reset doesn't work on Telstra phones

We’ve had persistent problems getting password reset SMSes via the Office365 administrator password reset functionality. Some to-ing & fro-ing with Microsoft support has eventually uncovered that their SMSes are being rejected by Telstra due to an ‘invalid format’.

They claim to be looking into it, but in the meantime if you’re an Office365 administrator and have a Telstra mobile, make sure you have a backup administrator account.