I know SMS infrastructure was built off of CiviMail but there is a key difference when building a recipient list from search.
Email - schedule/send via CiviMail > creates hidden group
SMS - schedule/send > puts contact's phone numbers in an array similar to "send email" to 50 or less option.
- If you use the
SMS - schedule/sendoption it does not show in
Find Mass SMSscreen https://example.org/wp-admin/admin.php?page=CiviCRM&q=civicrm%2Fmailing%2Fbrowse&reset=1&sms=1.
- We had a very high non deliverable rate (using Twilio) due to a Queue overflow error: https://www.twilio.com/docs/api/errors/30001. MPS rates for some orgs is only 1-3MPS, for us it is 75MPS but even so doing a search action mass sms to a good deal above your MPS will cause the Queue overflow error.