“Massive DDoS attack” just T-Mobile error?

Even if you weren't connecting through T-Mobile managed hardware, if the T-Mobile customer base found or was provisioned with alternative connection mechanisms then they could have been competing with you for bandwidth.

The direct answer is no, network outages don't effect services but if you can't access them the experience is though they were out. Consider a friend you want to call. They may be alive and well (or even besides you) but if your phone can't reach theirs you still can't talk to them on your phones.

- Facebook sends SMS 2FA. Users on T-mo would be unable to receive codes. What behavior does the 2FA-sending code have in this condition? Does the process just sit there and time out (possibly exhausting some finite server resource, like file descriptors)? Are there automated retries in a client? Are humans retrying aggressively because they are frustrated by being unable to log in?

- SWEs and SREs on on-call rotations may have configured pages to go to a cell phone on T-mo. Usually the three fallbacks are "phone call", "sms", and "pubsub/push notification", but both voice and data were broken today. May have taken longer to notice/escalate/resolve an outage.

- Messenger may have seen increased load because people were using it to replace voice and texts that they would have normally sent over T-mo.