Twilio is great, I used it some time ago and all good. I think wholesalesms.com.au may be cheaper, I’ve also used them and they’re very similar.
One slight drawback I can think of is that an API upgrade might force you to update your code too. These might be rare, but will have you reading old code which is always fun.
If you just want to send sms using a 4g mobile sim card, I’ve been using SIM7600E-H hats and they work really well. I’ve used them with Telstra and Optus around the country with no problems. They’re not cheap, but they just work. The GPS is pretty good too if there’s any need for it on your solution. Only thing is they’re quite power hungry, I’ve used Meanwell 5v 15w power supplies and they’re A-ok at around $20! No low power messages
There are wikis with example code to get you up and running in no time whether you use C, Python and/or AT commands. I’ve also used modem manager and qmcli and they work pretty well too.
I’d watch out for hats that use sims LTE-IoT like the SIM7000E. I couldn’t get them to work, probably needs a sim card on a M2M plan which you’d need a busine$$ account to acces$$. I’d also watch out for hats with only 2g technology as Australia shut that network off a while ago, they wont work.
Hope that helps.