Tutorials: Importing Just A List of Phone Numbers
Every carrier creates an email for their users (most people do not know this). You could create a form that concatenates the fields to give the signup person a distinct email. (they would still need to fill out a form)
The snippet of code will auto-create your contact with an actual working email - just in case someone accidentally sends an email to that contact. I use Amazon AWS for my sender and of course, I use Groundhogg in all of my personal projects. I tested this to make sure that it works. With that said, you will have to create the code specific to your form. Here is a little documentation to help you along the way.
This is a list of the @domain part of the subscribed carrier for the US and Canada
This is an example of how you would create the form (you would not need the username, profile picture, or about me fields)
The code you will need is going to be contingent on which form creator you are using (gravity forms, forminator, and such) - however, this will help point you in the correct direction.
https://yikesplugins.com/knowledge-base/contact-form-7-populate-merge-variable-data-with-additional-user-information/
If you do not generally work in code such as this or would like assistance I recommend you check our out partner's as I am sure one of them will be able to assist you.
https://www.groundhogg.io/partners/