User can send the message from a phone/mobile number which is saved in salesforce Contacts/Leads/Opportunities:
1. Send a message from a desired handheld device (cell phone, tablet or any other) to a salesforce saved Contact number (Contacts/Leads/Opportunities).
2. Now, login to the salesforce and select the Contact where message is sent.
3. Contact will show the message as Incoming message/Task under the Activity tab.
Note: For receiving messages make sure that the Site URL which is generated on the Company Information tab should be registered/saved to the connected Twilio account under the related phone number as shown: