Hi filipistrate ,
For the SMS to send out, users need to make sure the gateway always OPEN. But some of the android device will kill the background processing app which from my experiences, Zender app will be stop when there is no any action. With 5sec interval for 1 SMS, maximum 24hours can only send 17,280 SMS. If the site really need high volume and responsive SMS, better go with Twilio or such provider.
For WP / WC automatically send out SMS which is quite challenging. The gateway has to be standby 24 hours which technically still not possible to make it. The rest I will let Zender team to advice whether is it possible to achieve or not.
Of course I hope this can be done as well 😀