There are some Backdrop contrib modules dating back a few years.
Two that we started to test are the twilio and SMS Framework contrib modules.
The twilio module provides a simple setting page and an option to send a test SMS message that works.
The SMS Framework module is a collection of many modules including a sms_gateway module with a user interface that is difficult to navigate and may be broken.
As I understand, the drupal smsframework module does not include gateways. These are left for other modules like sms_twilio or twilio, since telephony providers may provide both voice and sms connectivity.
In terms of drupal module usage is over 400. There may be growing interest for this in backdrop.
Both of the mentioned backdrop modules provide integration with the rules and token modules and the SMS Framework appears to do the bulk of that.
For reference, there is another drupal project with emphasis on using tokens and views, https://www.drupal.org/project/smssystem
For backdrop, there are other related projects to keep in mind, https://backdropcms.org/project/webform_telephone
For general browsing and inspiration, some interesting blog posts:
https://blog.dcycle.com/blog/2022-04-19/phone-verification-drupal/
https://www.twilio.com/en-us/blog/connecting-all-with-voip-drupal-and-twilio-html
Comments
Not surprising after 8 years, there's room for improvements and things that come to mind are multiple phone numbers, multiple uses, and possibly interface with the trust settings and protocols required for telephony networks, or helpful links to the information.
I started issues in each of the contrib module spaces to get some feedback on where the interest may lie.
And just to bookmark it for later followup there's the https://github.com/backdrop-contrib/sms_clickatell module as an example of the interface for provider and the sms framework.
I may not have looked in the right places, I started looking for Backdrop pages equivalent to the Drupal site building pages, with discussion on using contrib modules to configure functionality, such as the twilio module.
A place where the discussion can be organized. Otherwise, it may be threads in the forum and/or scattered issues in various github queues.
And we can keep building on the knowledge shared over the decades through the drupal community in the site building guide.