Ask HN: How do you get in touch with your early customers?

I have built https://get.mymfa.io and managed to get ~75 active paid customers but so far I have not been able to get in touch with any of them to efficiently gather feedback.

I have tried getting in touch either through generic and/or personalised emails asking them to book a 15 minute slot in their agendas but I haven't gotten many interesting responses so far (~2-3 users wished to share some feedback).

How do you manage to engage with your customers and make sure you can setup a quick call to chat with them? Is there a more efficient approach like a questionnaire and offering discounts if they reply ?

Thanks! :)