[HWR_14]

Most providers let you email phone numbers to send an SMS. I don't know if your LEO phone would. However, when I attempted it every "from" was a different random provider phone number (fewer digits than normal)

Keep in mind SMSs are pretty rate/bandwidth limited in your use case. SMSs originally (and probably still in your case) were based on having 140 bytes of freeish data in the keep-alive style pings between tower and phone. So you only get/send one SMs per (on regular devices I believe this is no longer the case).

Can you program the device you will be using, or does it need to be human readable?

One solution might be to run a cloud SMS service. You can set up an SMS endpoint on AWS with bidirectional communication. It would let you write as complex code as you would like to generate/parse the SMSs, and probably falls into almost free at a personal scale.

As a warning as someone who built a little SMS autosender (as I said earlier based on email), be careful you don't accidentally DoS yourself. Consider some way to turn it off. Constantly getting texts on a device without the ability to assign a specific ringtone because the event you was sure was rare happened more frequently was no fun until I changed the code.

[moltke]

I've never had the email/MMS gateways work. Personally I prefer JMP.chat which gives you a nice XMPP/MMS gateway and is much easier to work with.

[HWR_14]

Note that the OP must use SMS, not MMS.

In the US it wasn't an issue. What was the trouble? Keep in mind I was sending it to one or two people. I have no doubt it doesn't work on a mass scale.

[moltke]

JMP.chat does pure SMS by default, you have to ask to be part of the MMS beta.

I've only really tried AT&Ts gateways in the US from gmail, maybe something has changed but it never worked for me.

[HWR_14]

AT&Ts gateway worked for me around 2019. I was sending it from a domain that was set up with all the anti-spam things you have to set up to be a good sender (whatever rules so GMail etc don't dump all your messages right to spam, they are recorded but I don't recall them). It's possible that GMail was blacklisted because a lot of spammers had a ton of Gmail accounts.

Edit: Super important. You can turn that feature off per phone number. You (or your recipients) may have told att not to send them