See a sample page here.
Note: When describing messaging frequency, a statement like "you can expect to periodically receive text messages" is not enough. You must set a clear expectation with a specific number of texts, for example: "You can expect to receive no more than 1 text per day." or "You will receive a maximum of 10 texts per week."
Image above: toll-free opt-in pages must include an optional field to enter a cell phone number, a checkbox to affirm opt-in for SMS, and a call to action explaining to the user who will text them (the sender), how often (frequency), how to get support and opt-out, and that message and data rates may apply.