If you want to use this as a toy, sure. But please don’t rely on it for messages you want to actually get delivered: These gateways frequently drop messages for random reasons, or block your IP if you send more than a few messages at a time.
Also, in countries other than the US, expect the gateways to be long gone, or completely unreliable.
I work with HVAC software and so many of our customers want important alarms on their mobile devices. They don't want it in email because nobody has notifications on for email so they want a text and they are aware of the carriers SMS gateway. Five years ago this worked ok but with the rising spam wars these similar alerts we send out get caught up in spam filters easily and customers blame us.
It is very simple to use something like pushover and API alarm delivery to them, and so much more reliable. Still it's shocking to see; wait that requires us to pay pushover? Nah, we'll take our chances with email@SMS for our multi million dollar chiller plant in our hospital alarm notifications.
Exactly. That's why you don't want to use these free open relays, ever, for anything that you actually want to get delivered. There are a lot of companies out there that specialize in SMS delivery and have built really smart/complex route selection, retry, failure handling and cost optimization logic to get to a reasonably high delivery %.
/spam: my friend runs https://messente.com that specializes in global SMS delivery and they are, honestly, quite good at it. (Let me know if you ever start using them so I can let him buy me a referral-beer at the pub)
