When you create a poll, each of the options are attached to a certain number starting from 1.
To vote, your audience needs to send one of the numbers (for example 2) or the option itself to your dedicated phone number in an SMS.
Here's a poll for example:
Which is your favorite fruit?
To vote for Banana, text 1 or banana to the phone number.
After the vote is accepted, the voter will receive a confirmation text message which can be customized.