Copied directly from the Survey Sampling.com website, which other commenters have linked to this phone number:

"Opt Out from Survey Phone Calls

We appreciate your participation in our research and we value your opinion. But if you do not wish to participate in future surveys, we understand and respect your decision. You may choose to opt-out through any of these means:

Email us at [email protected] and include your phone number in the body of the email.

Call 1-866-360-9563 from within North America or 1-801-379-4094 outside North America to be placed on our no-contact list.

To speak to someone about the call you received, contact us toll-free within North America at 1-866-360-9678 or call 1-801-379-4094 outside North America."