[przeor]

Collect phone numbers during the registration process with Twillio veri codes. Then ask them for feedback via phone number.

You just need to justify during registration, why the user will benefit from leaving the number during registration. You can write it down and half of the interested user will leave the number that you can collect feedback from more efficiently.

[RamonMamon]

That sounds like a great idea! As long as it's not a mandatory field, and properly justified and incentivized, then it sounds like a good way to find willing research participants.

Out of curiosity, couldn't the same result be achieved with an email address instead?