How do SMS messages work?
SMS messages are a great way to contact hosts and visitors within Sine.
When a visitor arrives on site and selects their host, a notification can be sent via SMS, email and/or push notification to the Sine mobile App. These notifications are included on all plans at no additional cost.
Bulk SMS is not supported by Sine. Instead, messages can be sent to multiple visitors or hosts at once via email or push notification. See more here