The SMS button appears when you are looking at a single Projects, provided you have authorisation to send SMS from OpenCRM (see this FAQ for setting this up).
When you select the SMS button you are taken to the SMS create screen. Any Contacts that are linked to this Project are linked to the SMS - if the Contact does not have a mobile number they will not be listed.
Selecting SEND will send this SMS to all of the contacts listed.
Need More Help?
If you have any questions, just start a Live Chat, "Click" on the Chat Icon in the lower right corner to talk with our support team.