Years ago we set up our own SMS gateway to send Transactional SMS messages to NZ and AUS. Our system is three Android phones with a dedicated app.  3 Phones gives us redundancy. iPhone's are not an option because you cannot programmatically send an SMS message from an IOS APP.

We built our own gateway because we found paid/cloud gateways to be unreliable (max 95% delivered) and expensive at around 16cents a text.  Sending SMS this way is in breach of the telco's T & Cs, but it is the most reliable way I have found to send SMS.  

With the 3g networks being turned off, we have upgraded the mobile phones to 4g phones.  Since then, the system has become less robust as newer Andriod phones are more fussy about long running apps and power management.

Any advice on where to from here is appreciated.  If you send SMS mainly to NZ and AUS, which provided to you use or have you made your own gateway?