Network providers usually come up with a report in case of message delay or delivery failure.
Are these three users having trouble with delayed messages from each other? The delays so far have been up to a half hour.
The pattern is I do not receive anything form the sender until another message is sent by the same sender, or I receive a call from the same sender.
I called T mobil's customer service and they mentioned changing the settings on their end so I won't miss call in future but I'm skeptical based on above responses.