That number is set by the company that provides our text messaging gateway, which is how we receive participant text messages into Poll Everywhere. If your audience is curious, just tell them not to worry. It is not a toll number and it will not bill anything besides standard rate text messages to a participant's mobile phone bill. Standard rate text messages in the US usually range from free or $0.01 per message up to $0.20 per message for people who do not have a text messaging plan. Check with your mobile carrier for current pricing.
You can also view the other available text to numbers on your settings page, under International Text Messaging.