I have a SwitchBox (https://switchboxcontrol.com/)
This is a box that uses a T-Mobile SIM to control power to remote devices. It is used typically in an aircraft hangar to turn on an engine pre-heater. It works by receiving SMS messages containing commands. Sometime when I send it a command, it responds within a few seconds, and sometimes the message is not delivered for a few hours. This afternoon one message took 4 minutes to be delivered to the box and the next one took two and a half hours. In both cases I can see the credit debited from my pre-paid T-Mobile account immediately.
What is the problem with these long delays when sending an SMS to a T-Mobile device?
Best answer by tmo_chris
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