Can I send an inbound (text-in) poll to phone numbers I've collected?

No; currently, an inbound (text-in) poll cannot also be sent out directly to participants' cell phones.

To send your inbound survey questions to phone numbers you've collected, create a second poll with the same questions as your inbound survey, and set it to Outbound ("Send the poll to participants") in the poll's Publishing Options. Each poll will have its own Poll Dashboard, and the response data will be kept separate. However, if you have used the same sequence of questions in both polls, you can easily merge the data sets for analysis when you download the results at the end of your campaign.