MFA phone verification fails sometimes or Microsoft 365 did not send me a code?
A U.S. system always produce these calls with a +1 caller ID.
The carrier within this connection likely doesn’t support dual tone multi frequency (DTMF). Therefore, MSFT never receive the # signal back from the user’s phone. As a result, failure of authentication may be encounter.
In other words, calls could get routed through a carrier between the source and target that doesn’t support or pass along the caller ID that MSFT sent.
If you attempt again within 10 minutes,
MSFT will choose a different PSTN provider to ensure a different routing and increase the chances of a successful connection.
If you are on a specific country that are having a number of MFA phone verification fails intermittently,
By logging a support case, MSFT could run a deliberation to see if there’s a need for further fix within your location otherwise choose other methods such as the Authenticator mobile app or text message.
In conclusion, MSFT has given multiple options you can choose from for your best case scenario. More on securing Microsoft 365 here.