Twilio

Register on https://www.twilio.com/try-twilio.

Insert your phone number to complete the registration.
Now click on “Account” as shown in the image below to recover the credentials requested by the plugin.

Copy the values of “ACCOUNT SID” and “AUTH TOKEN” fields and paste them in the plugin option fields. According as you are in the test phase or not, the service offers two pairs of different keys.

Among the available services, Twilio is the only one that verifies the sender number you decided to show to users.
For this reason it is essential to purchase a number at the following link: https://www.twilio.com/console/phone-numbers/dashboard
Now you can insert the number in “Sender telephone number” field.